Why is 'shucks' (as in 'aw, shucks') used with an '-s' ending?

Solution 1:

Etymonline lists the origin of shucks thusly:

Interjection shucks is 1847, from sense of "something valueless" (not worth shucks).

So, the -s termination comes from the plural in the original, longer expression.

Solution 2:

'Not worth shucks'

John Bartlett, Dictionary of Americanisms, first edition (1848) has this entry for shuck:

SHUCK. The outer husk or shell of the walnut, chestnut, &c.; or the husk of Indian corn. In England, the word is applied to pods as well as husks; as, pea-shucks. Not worth shucks, is a Southern expression meaning good for nothing.

[Example:] If them thar is all he's got to offer, he aint worth shucks; and if you don't lick him you aint worth shucks, neither.—Robb, Squatter Life.

[Example:] They had three or four hounds, and one great big yellow cow, what wasn't worth schuks to trail.—Maj. Jones's Courtship, p. 48.

The Robb quotation is taken from John Robb, Streaks of Squatter of Life, and Far-west Scenes: A Series of Humorous Sketches Descriptive of Incidents and Character in the Wild West (1843). The expression "worth shucks" actually appears twice in that book—first in "Telegraphing an Express," a story set near East St. Louis, Illinois, where three men from St. Louis, Missouri (on the other side of the Mississippi River) charter "a wagon and horses belonging to a couple of suckers [citizens of Illinois]":

"They intend to steal sum gal on the road," whispered one sucker to his friend.

"Well, they've got cussed poor taste, fur I'll swar that aint anythin' on this yeur road to the bluff wuth shucks, 'cept Nancy Birch, and her temper would tarn the stomic of the d———l."

The second, "Yaller Pledges," appears to be set in central Missouri near the Osage River:

"'The nasty dog,' ses Sally, 'does he think I'm agoin to nuss any of his yaller pledges—ef them thar is all he's got to offer, he aint wuth shucks; and if you don't lick him fur his onmannerly note, you aint wuth shucks, neither.'"

(This story turns on a fight between two suitors for Sally Spellman's affections. One—a man from Louisiana—makes the mistake of offering, in a note to her, "to bring a pledge of affection from the sunny south, to bind our openin' loves." Evidently a "yaller pledge" was a slang term in Missouri for a mixed-race child fathered on a black slave woman by a white man.)

Major Jones's Courtship is an invention of William Thompson titled Major Jones's Courtship: Detailed, With Other Scenes, Incidents, and Adventures, in a Series of Letters, and set in the town of Pineville, Georgia. The book was was in a twelfth edition by 1852, and in that edition the "cow" is a cur and "schucks" is spelled shucks:

Cousin Pete was long, with two hound pups, and Tom Stallins had three or four hounds, and one grate big yaller cur, what wasn't worth shucks to trail, but was bomination to fight. Ben Biers had more dogs than you could shake a stick at ; and sich another hellabaloo as they all made! why one couldn't hear himself think for 'em.

Another early occurrence of "worth shucks" appears in Thomas Thorpe, The Taylor Anecdote Book: Anecdotes and Letters of Zachary Taylor (1848) and involves another Missourian, this time a recruit joining the Army of the West in St. Louis (presumably in 1846, just prior to the start of hostilities in the Mexican-American War of 1846–1848):

The colonel [Kearny, later a general in the war] tried to look grave at this familiarity [being greeted warmly, slapped on the shoulder, and invited to have a drink by ten new recruits], but it was done by those who were so evidently unconscious of any breach of etiquette, that he was forced to laugh, and humored them by taking a glass of wine with them at the bar—the tall boy [from Missouri] telling him at the same time, that his drink "warn't worth shucks, and only fit for wimen."—"Why in thunder," cried another, "don't you go the corn-juice, general, it's the only stuff for a military feller to travel on?"

But earlier than any of those instances of the phrase is the one from this fictional dialogue originally from the Mobile [Alabama] Register, reprinted in the Richmond [Virginia] Enquirer (June 30, 1840):

Bubblecurency ["a hard cider speculator"]. Awful times! awful times. This abominable admistration will be the destruction of every body.

Dem[ocrat]. How so, neighbor Bubblecurrency?

B[ubblecurency]. How so! The blindness of some people is amazing! Why look at flour, it only fetches live dollars a barrel; and cotton is not worth shucks.

'Wouldn't hold shucks' and 'whipped into shucks'

That shucks wasn't understood throughout the United States in the 1830s to refer to natural coverings on ears of Indian corn is clear from this in-text occurrence and explanatory footnote in "Buck Horn Tavern, a Scene in the West," in The Knickerbocker, or New-York Monthly Magazine (January 1834):

—I heard something which sounded like the rustling of shucks, and in a few minutes after every thing was as quiet as the wild woods—

'Shucks,' the husk of corn.

The scene takes place in western Tennessee, which gives you a sense of how geographical notions of "the West" have changed over the past two centuries.

Google Books and Elephind searches identify other early idiomatic phrases that include the word shucks. From "The Ring Tail Panther," in the [Brookville] Indiana American (August 31, 1838):

"There, now, is h—ll!" said he. "Mr. Speaker, Duff are sloped and we must expone the question till to-morrow; and if Duff don't stand up to the rack a little better I'll lick him, there's no mistake. If he slopes off in this way again when I want him, his hide won't hold shucks in two minutes arter, and I wont vote with him to copperate his Macademy.

From T. Egerton Brown, Trial of Judge Wilkinson, Dr. Wilkinson, and Mr. Murdaugh, on Indictments for the Murder of John Rothwell and Alexander H. Meeks, in an Affray Which Occurred at the Galt House, Louisville, Ky., on the 15th of December, 1838 (1839):

They had gone, it seems, too, in sufficient numbers to authorize the classical boast of Mr. Johnson, "that if they (meaning the Mississippians) came down their hides would'nt hold shucks."

And from Thomas Mayne Reid, The Scalp Hunters, in The Athenæum (July 19, 1851):

It required all the strength of nerve which the trapper possessed to conceal his chagrin. Without saying a word, he commenced wiping out his gun with that stoical calmness peculiar to men of his calling. I observed that he proceeded to load with more than usual care. It was evident that he would not rest satisfied with the trial already made, but would either beat the "Injun," or be himself "whipped into shucks." So he declared in a muttered speech to his comrades.

...

"Billee's right, Cap. If them Injuns must be fit—its got to be did whur thur's rocks or timmer. They'd whip us to shucks on the parairer. That's settled. Wal; thur's two things: they'll eyther come at us—if so be, yander's our ground"—here the speaker pointed to a spur of the Mimbres—"or we'll be obleeged to foller them. If so be, we can do it as easy as fallin off a log. They ain't over leg-free."

I'm not sure what to make of the phrase "wouldn't hold shucks," though it's tempting to see it as suggesting "would be so full of holes that they couldn't be used to carry shucks in." The phrase "whipped to [or into] shucks" seems more likely to refer metaphorically to being beaten until nothing was left but the husk or shell. In any event, Google Books finds only one instance of each of these expressions during the period 1800–1870, whereas "not worth shucks" receives considerable use during the same period.

Another interesting development is the use of shucks in the South during the late stages of the Civil War to refer to Confederate money. From The New Eclectic Magazine, volume 7 (1870) [combined snippets]:

The jokes in regard to the Confederate currency are numerous and somewhat biting; the terms few, but significant. The notes which at the outset were dignified under the title of "bluebacks," eventually came to be known as "shucks"—"Confed-shucks," quasi "corn-shucks," sufficiently significant of their evil repute as a circulating medium.

'Tom Sawyer' and the emergence of stand-alone 'shucks'

The earliest Google Books match for "Shucks" as a stand-alone exclamation is in Mark Twain, The Adventures of Tom Sawyer (1876), where it appears nine times, usually by itself though once as part of the longer phrase "ain't shucks":

"Oh, shucks, I'll be just as careful. Now lemme try. Say—I'll give you the core of my apple."

...

"Well, what is it? They'll all lie. Leastways all but the nigger. I don't know him. But I never see a nigger that wouldn't lie. Shucks! Now you tell me how Bob Tanner done it, Huck."

...

..."I been to the circus three or four times—lots of times. Church ain't shucks to a circus. There's things going on at a circus all the time. I'm going to be a clown in a circus when I grow up."

...

"O, shucks! Baby! You do want to see your mother, I reckon."

...

"What a curious kind of a fool a girl is. Never been licked in school! Shucks, what's a licking! That's just like a girl—they're so thin-skinned and chicken-hearted. ..."

...

"Shucks, I only meant you'd see 'em—not hopping, of course—what do thy want to hop for?—but I mean you'd just see 'em—scattered around, you know, in a kind of a general way. Like the old hump-back Richard."

...

"Shucks, witches ain't got no power in the daytime."

...

"But shucks! Your mother won't know, and so what's the harm? All she wants is that you'll be safe; and I bet you she'd 'a' said go there if she'd 'a' thought of it. I know she would!"

...

"Shucks, what do you want to slope for?"

Twain was born in 1835 and grew up in Hannibal, Missouri, which is about 100 miles northwest of St Louis. It seems probable that he was familiar with the expression "not worth shucks" from his earliest childhood.

An Elephind search, however, finds instances of stand-alone"Shucks" from as early as 1845. From an item in the Edgefield [South Carolina] Advertiser (October 28, 1846), originally printed in the Marion [South Carolina] News:

Mr. Shuck, the Missionary; and Miss Sexton, were married last night in the Baptist Church, and we heartily wish that in the future enjoyment of their matrimonial sweets, they may not have occasion to exclaim "Oh Shucks!"

And from "Shakspeare Readings--Hamlet," in the [Okolona, Mississippi] Prairie News (January 20, 1859), originally from the Brandon [Mississippi] Republican:

Pol[onius].—Well, I'll tell you—if you don't mind, he'll make a fool of you.

Oph[elia].—Why, daddy, he told me he loved me powerful, sure.

Pol[onius].—Oh, shucks! just let me tell you, once for all, he's a wild, rattling, rip-snortin' hifalutin fellow, and would just as soon tell you a lie as the truth, and don't you never believe nary word he says—d'ye hear?

Oph[elia].—Yes, daddy, and I'll mind you.

Conclusions

It seems very possible that the slang term shucks had its origin in the idiomatic phrase "not worth shucks" (or "ain't worth shucks")—perhaps influenced by other idiomatic references to shucks as worthless husks—from the first half of the nineteenth century, in the U.S. South and Midwest. The phrase "not worth shucks" had roughly the sense "not worth so much as the outer shell of a walnut or the outer husk of an ear of corn."

In the source material that I've been able to locate, the earliest instances of various shucks-related expressions are as follows:

won't hold shucks: 1838

is not worth shucks: 1840

oh shucks: 1846

whipped into shucks: 1851

Shucks as a stand-alone expression first appears in Google Books search results in Mark Twain's hugely popular novel Tom Sawyer, published in 1876, but it was in use in the South at least thirty years before that, as recorded in newspaper articles in the Elephind database.

It is certainly possible that the choice of shucks as the object of comparison to indicate worthlessness was euphemistic, with a much coarser object of comparison implied. But none of the sources I consulted make this claim.