How did "sweet tooth" originate?

Given it's nearly Easter, I was eating some fruity Jelly Beans (I had forgotten how good they are) and my Indian friend said that he loved them because he had "sweet teeth".

I corrected him, saying 'for reference we'd usually say, "I have a sweet tooth"', but I couldn't think of a reason why besides convention.

I found it in some old books from the 17th century, but some other sites claim it comes from the 14th century.

I realize the term refers to a person's fondness towards sweet tasting foods, but how did it start to be so? And how come it's not plural?

From an idiomatic perspective, the expression was coined during the Middle Ages with the simple combination of two common terms. The second term, tooth, was already used idiomatically meaning 'taste, liking'.

Sweet tooth (n.) :

• "fondness for sugary stuff," late 14c., from sweet (adj.) + tooth in the sense of "taste, liking" (see toothsome).

Toothsome: (adj.):

• pleasant to the taste," 1560s, from -some (1) + tooth in a figurative sense of "appetite, taste, liking" attested from late 14c. (compare sweet tooth, also figurative use of palate). The extended sense of "attractive" (1550s) is attested earlier.

(Etymonline)

From a scientific perspective the origin of our perception of sweet vs bitter tastes has a much older story:

The Evolutionary Origins of the Sweet Tooth

• The evolutionary explanation for the sweet tooth revolved around that idea that we have physiologically associated a sweet taste with high-energy foods which would have helped our earliest ancestors survive better in their environment (getting more “bang-for-the-buck”….if an individual has to spend time and effort foraging for food, it’s better to obtain energy-dense food items than energy-poor food items). When one considers our ability to taste, our ability to perceive “sweet” is relatively weak, while our ability to perceive “bitter” is generally considered much stronger (in fact, the strongest of our taste reception, on average). Perception of “bitter” is thought to be an evolutionary strategy of quickly identifying plants that contain potentially harmful toxins (produced as secondary plant compounds). Thus, evolving a low tolerance to “bitter” and a high tolerance to “sweet”‘ might have promoted our ancestors to actively seek out sweet tasting foods.

(Interview at The Smithsonian, evolutionary biologist Jason Cryan )

Let me answer your last question first: a sweet tooth is a synecdoche, a figure of speech in which "a part is used for the whole or the whole for a part" (Webster's Dictionary, New York 1994, or online).

I can't provide a sound derivation for that idiom, as the sources available to me (easily, I'm sorry I can't comb through the 284,000 Google results) with etymological information about it are pointing to the 14th century as to its origin, provide the meaning we're referring to today (see, e.g., OED), but don't explain if this really was the first and original meaning.

(As to dating, let me tell you that finding a book from the 17th century containing the term in question doesn't mean the term originated in that period while other sources claim an earlier date. What you find on the first three pages of an ad-hoc internet search is never the big picture.)

But let us take a closer look at that source you found, the Nouvelle Facile Methode Pour Apprende l'Anglois, from 1698. The portion of the text containing our sweet tooth quote reads as follows:

The great Mutton is commonly course [coarse], and is not for a sweet Tooth. – But the small Mutton (or rather the middle size) such as feeds in dry Pastures, is very palatable.

Sweet doesn't necessarily mean tasting like sugar but can also mean fresh, pleasing, delicate, agreeable, dear, or easily managed. Now that we have the pair of opposites: not for the sweet tooth/palatable referring to mutton it seems highly improbable that sweet should mean sugarlike in this context, but, as palatable suggests, rather something like pleasing or delicate.

Now mind the left column providing a French translation of the English text:

Le gros Mouton est generalement grossier, & ne se mange guere parmi les honnêtes gens. – Mais le petit Mouton, qui pait dans les Lieux secs, a un Gout fort agreable. [The big mutton is coarse in general and is hardly appreciated among noble people. But the small mutton, feeding on dry pastures, has a very pleasing taste.]

This gives us a hint what people having a sweet tooth might have meant originally: People who are very delicate and paticular about what they eat, implying people from the upper classes who could afford being delicate.

Searches of Early English Books Online, Google Books, and Hathi Trust produce several dozen unique matches for "sweet tooth" (or "sweete tooth") from before 1700, including thirteen from before 1620. Here are those earliest thirteen, in chronological order, which I submit in hopes that it may give a sense of how the term was understood during the period 1575–1620.

From Heinrich Bullinger, An Exhortation to the Ministers of Gods Woord in the Church of Christ, That They Set Aside All Mutuall Discord, and in These Latter Dayes and Dangerous Times Purely and with One Concent Preache vnto the World the Onely True Faith in Christe and Amendement of Life (1575):

Secondly, a sweet tooth, and a fare mouth, that is, daintinesse, or choicenesse in diet is an enimie to frugalitie, a needlesse charge, to de∣light the taste for a moment, whereas wholesome meat and drinke, would be more ease for the purse, and more healthfull for the bodie. He that loueth Wine and Oyle, that is, sweet delicats for his sences, will not be rich Pro. 21. 17.

From John Lyly, Euphues and His England Containing His Voyage and His Aduentures, Myxed with Sundrie Pretie Discourses of Honest Loue, the Discription of the Countrey, the Court, and the Manners of That Isle (1580):

Wine is the glasse of the minde, and the onely sauce that Bacchus gaue Ceres when he fell in loue: be not daintie mouthed, a fine taste noteth the fond appetites, that Venus said hir Adonis to haue, who séeing him to take chiefest delight in costly cates, smyling said this.

I am glad that my Adonis hath a swéete tooth in his head, and who knoweth not what followeth. But I will not wade to farre, seing heretofore as wel in my cooling card, as at diuers other times, I haue giuē thée a caueat, in this vanitie of loue, to haue a care: & yet me thinketh the more I warne thée, the lesse I dare trust thée, for I know not how it commeth to passe, that euery minute I am troubled in minde about thée.

From M., Io., Phillipes Venus. Wherein Is Pleasantly Discoursed Sundrye Fine and Wittie Arguments, in a Senode of Gods and Goddesses, Assembled for the Expelling of Wanton Venus, from Among Their Sacred Societie (1591):

Cyane was stung with Venus tale, and though at firste she began to stand, yet bethinking her selfe better, she recouered her strength, & because none should know where the showe wrong, but she that wore it, with this prittie glose plaied on her game: I haue fed Venus (said she) too rife on Dorys reasons, so sweet is her fruite as if she bore foorth Figges: Héereat Dorys thought to catch Cyane at the nyck, requiting her curtesie with this prittie quippe: Cyane (answered she) euer caryeth a sweete tooth in her head, and a swéet thought in her hart, beauty bréedes no other, loue as much.

From John Lyly, Midas Plaied Before the Queenes Maiestie vpon Twelfe Day at Night, by the Children of Paules (1592):

Petulus. O monstrous mouth! I would then it had been a sheepes eye, and a neates tongue.

Licio. It is not for the bignes, but the sweetnes: all her teeth are as sweet as the sweet tooth of a calfe.

Petulus. Sweetly meant.

From Robert Cleaver, A Briefe Exiplanation of the Whole Booke of the Prouerbes of Salomon (1598):

Vers. 8. The morsell which thou hast eaten thou shalt vomit vp, and lose thy sweete words.

As a little before hee aduised vs not to be too desi[r]ous of the delicates of great men: so here he counselleth vs to beware, that we be not too much beholding for foode, or any other benefits, to misers and churles. Eate not, nor partake of, without consideration, or iust warrant, the bread, drinke, or any kinde of foode, or other beneuolence, of him that hath an euill eye, of a too much restrained or niggardly person, such a one is contrary to him that is liberall, and hath a good eye, chap. 22. 19. neither be desirous of his daintie meates (because a sweete tooth, and hungrie appetite, leade men many times to vnfit places) For as he thinketh in his heart so is he, he is not as he pretendeth with his tongue, and as he maketh semblance of with his countenance, tranke, and free, and glad to giue entertainment: but miserable, and churlish, because he thinketh all lost, that any man hath from him, may that goeth out of his dish, or out of his cup, or out of his purse, although it be for his owne people, or his owne lips: he begrudgeth himselfe sufficient of foode and necessaries, and how can hee then willingly spare any thing to strangers? he cannot affoord his own belly a good morsell of meate, and would it not grieue him that thou shouldest eate vp his victuals: and therefore though he say vnto thee, eate drinke, goe to I pray you, spare not, you are welcome, &c.

From Thomas Dekker, West-ward Hoe As It Hath Been Diuers Times Acted by the Children of Paules (1607):

Mistress Honnysuckle. Sir Gozlin? I doe take it your legs are married.

Sir Gozlin. Why mistris?

Mistress Honnysuckle. They looke so thin vpon it.

Sir Gozlin. Euer since I measurd with your husband, I haue shrunk in the cal[f]e.

Mistress Honnysuckle. And yet you haue a sweet tooth in your head.

Sir Gozlin. O well dealt for the Calues head, you may talke what you will of legs, and rising in the small, and swelling beneath the garter. But tis certain when lank thighes brought long stockings out of fashion, the Courtiers Legge, and his slender tilting staffe grew both of a bignesse. Come for Brainford.

From Joseph Hall, Pharisaisme and Christianity Compared and Set Forth in a Sermon at Pauls Crosse, May 1. 1608 (1608):

The great Doctor of the Gentiles long ago said, All seeke their owne, and not the things of God; and is the world mended with age? would God wee did not find it a sure rule; that (as it is in this little world) the older it growes, the more diseased, the more couetous: we are all too much the true sons of our great Grandmother; and haue each of vs an Eues sweet tooth in our heads, we would be more than we are; and euery man would be either [untranslated words in a non-Latin alphabet] or [untranslated words in a non-Latin alphabet] either the man, or some-body.

From John Dod, "The First Sermon of the Lords Supper" in Ten Sermons Tending Chiefely to the Fitting of Men for the Worthy Receiuing of the Lords Supper (1610):

First, the causes of this hiding of sin are naught : which are these following.

1 The first, is the loue of iniquitie: for sinne is of that nature, that it will neuer tarie but where it is loued and much made of: it is such a guest as rough intertainment would driue away in a short time.

And that such doe loue it, it is very euident in that former place of Iob: where it said, that first, wickenesse is sweet, and then they hide it. Looke what appetite and eager desire any one that hath a sweet tooth can possibly haue after pleasant meates and dainty dishes: the same or greater haue they after sinne: the loue whereof must needes be odious, because it is Gods vtter enemie, and therefore the hiding of it must needes bee dangerous.

From Robert Hill, The Pathway to Prayer and Pietie (1613):

Quest. You said in the third place, I must auoid those things which are enemies to frugalitie, which are they?

Answ. 1 Sloth: which is described to be a great wisher Pro. 13.4. excuser Pro. 15.19.22.13.26.14.15. ouer-wise Pro. 26.16. & the high way to beggery. Prou. 20.4.10.4.24.24.36.

2 Vaine & idle company Prou. 28.19. these wil driue you either to other delights, or to loose your time or to let go occasions for your good; be you neuer so good, bad company wil hurt you, as the swéetest waters poured in∣to the sea become salt, and brackish.

3 Take héed of pastime: Prou. 21.17.

4 Of talking what you wil do Pro. 14.23.

5 Of a swéet tooth, & a veluet mouth, which often procureth double expences: first of di∣et, secondly of physicke to cure diseases got∣ten by intemperance. Prou. 21.17.23.21.29.

From a 1615 translation of Benedetto Varchi, "The Fruits of Jealousie," in The Blazon of Jealousie: A Subject Not Written of by Any Heretofore (1615):

So bankets I (oft) made to thee, / Such stuffe as with the Grosers be, / To dainty Dinners often, I / Inuited thy sole compny; / So oft with costly Suppers sweet, / And Breakefasts fine I did thee greet, / Preferues, Conserues, and fuch like stuffe / As Ciuet, Muske, thou hadst enough: / So haft thou now sweet tooth too sweet, / And liquorish mouth for thee most meet.

From The Owles Almanacke Prognosticating Many Strange Accidents Which Shall Happen to This Kingdome of Great Britaine This Yeere, 1618 (1618):

GROCERS.

Neuer looke as pale as your sugar-loafes (you cinnamonian Gingibers) for that your spices grone in their bagge•… like a pigge in a wallet, I can tell you there are ten thousand sowre countenances, that hope all to be sweetned by the Grocer: Nay all the scoulds tongues in the Countrey that were wont to raile so bitterly must be bathed (as it is decreed by the authoritie of their Husbands) in the oyle of your ware, that is in the syrupe of suger; for your hotter spices, why they'le flie quickely, abundance of chollericke complexions will neuer bee without hot mouthes. And you your selues know that euery body will take pepper in the nose before he hath a casket to put it in. All the children in the world (if they be like me (will haue a sweet tooth in their head, the first that growes out of their gummes: Yea and your owne Prentises, will yerke a clod of Curranes currently downe their throates, and it may be pocket vp an iniurie as bigge as a pound of suger to welcome a friend in a Tauerne.

From The Happines of the Church, or, A Description of Those Spirituall Prerogatiues Wherewith Christ Hath Endowed Her Considered in Some Contemplations vpon Part of the 12. Chapter of the Hebrewes (1619):

Thou art a Christian, and fearest not that euer thou shouldst apostate into the deniall of thy Sauiour: yet let me say thou hast the materialls of this sinne within thee, timorousnesse and selfe-loue. Thou saiest, Sure I shall neuer be a drunkard, that belluine folly shal neuer apprehend me: yet thou hast the materialls of this within thee, and that naturally and hereditarily from thy first Grandmother Eue: a sweet tooth in thy head, a liquourish appetite to delicate meates, and intoxicating wines.

And from William Whately, A Bride-Bush, or, A Direction for Married Persons (1619):

But if her fondnesse and vanity bee such, that shee will needs exceede either one or both of these [a fair allowance to purchase necessities and her lawful liberty], either going an hunting after the fashion, or vying with some others (as undiscrete a body as her selfe) who shall be finest, or the like : then doubtlesse the authoritie and discretion of the husband must sound retraite to her pride and lauishnesse. He must neither weaken his owne estate, nor discredit his owne name, nor giue an offence to other, nor suffer her to pull reproach vpon her selfe, and misery vpon her family by ouer-pranking her carkasse, or ouer-much care to satisfie a sweet tooth, and a diſorderly appetite.

Discussion

Some of the examples (particularly the two from Midas and Westward Hoe, which involve repartee and share a connection between "sweet tooth" and calves) are difficult to make sense of. Others, however, show a couple of tendencies in early usage.

One is to equate "sweet tooth" not specifically with honey, sugar, or other sweets, but with a taste for dainty, expensive, and unwholesome foods and (more generally) for an unsupportably extravagant life style.

The other is to equate a "sweet tooth" with sinfulness. In this regard it is interesting that both Joseph Hall (in 1608) and Thom,as Adams (in 1619) explicitly connect the human tendency to have a "sweet tooth" with Grandmother Eve—the first and most disastrously unrestrained disorderly eater.

Solomon makes multiple appearances in early references to "sweet tooth" as well, but less to accuse sweet-toothed people of moral failings than to warn them as a practical matter that their habits will prevent them from prospering.

The only instance among the first thirteen that explicitly connects "sweet tooth" with a love of sugar seems to be the 1618 Owles Almanack instance, which notes all children's tendency to have a sweet tooth and then alludes to Currants and a pound of sugar. A slightly later instance is even clearer. From Gallobelgicus, Wine, Beere, Ale, and Tobacco: Contending for Superiority: A Dialogue (1630):

Sugar. Alas sir, like will to like, Sugar being of his owne nature sweete, has reason to make much of women, which are the sweetest creatures.

Wine. But some of them are sower enough.

Sugar. I sir, Widdowes at fifteene, and Maides at twentie fiue; but I keepe them company, for no other thing, then to conuert them, some of them could eu'n eate me, but for feare of spoiling their teeth.

Wine. Indeed one of your sweet hearts complained t'other day you made her teeth rotten.

Sugar. Alas sir, twas none of my fault, she bit me first, and I could doe no lesse, then punish her sweet tooth.

By 1630, then, "sweet tooth" was understood in at least some contexts as referring to "a taste for sweets" (and potentially leading to tooth decay). But its earlier meaning was more nearly "a taste for delicacies ruinous to health and fortune," which reductionist preachers often enough converted to something like "a taste for sinful self-indulgence."