Origin of "he's 6 feet tall if he's an inch"
I have heard this pattern used before in American English:
She's 6 feet tall if she's an inch.
It was a gallon of blood if it was a drop.
The baby was 10 pounds if it was an ounce.
I assume that it means something like, "She is 6 feet tall, which is very tall." But this to me is such a bizarre and illogical way of conveying the message that it's distracting, and I cannot get myself to say it even if it comes to me. Anyone know the origin of this, or can breathe some reasoning into this phrase to justify its appreciation?
I'll repeat what I said in the comments.
Firstly, the meaning of "She's 6 feet tall if she's an inch" is not "She is 6 feet tall, which is very tall", but "She's definitely 6 feet tall" or "I'm very sure she's 6 feet tall". That is, it's an emphatic version of "She's [at least] 6 feet tall", with the emphasis being on the truth of the statement, not necessarily the great height. As such, in usage it typically (but not always) follows a statement made by someone else (borrowing from mgkrebbs):
"She looks 5' 10'', don't you think?
Five ten?! She's 6 feet tall if she's an inch!
The purpose here is not just entertainment, but rhetorical effect: it's saying "I'm as sure of her being 6 feet tall as I'm sure of her being at least an inch tall" — and since I am obviously very sure she's at least an inch tall, I'm also very sure she's 6 feet tall. It's equating her being 6 feet tall to her being an inch tall: saying (not necessarily with justification, but that's rhetoric for you) that if you deny that she's 6 feet tall, you must also deny she's at least an inch tall, which you obviously cannot do.
The use of "logic" in rhetoric
More generally, the use of such implication sentences in rhetoric seem (to me) to be of two kinds. If you want emphasise a statement X (say emphatically that X is true), you can pick either the form
- "If T, then X" (= "X if T")
or - If not X, then F (= "X or F")
where T is some trivially true statement, and F is some trivially false statement. Examples of the "X if T" type include the ones you gave:
She's 6 feet tall if she's an inch.
It was a gallon of blood if it was a drop.
The baby was 10 pounds if it was an ounce.
She's forty if she's a day.
If I've told you once, I've told you a hundred times.
If I'm not hallucinating, I heard you come in at 3 am last night.
Examples of "If not X, then F" type include:
If he's a doctor, then I'm the Pope.1 (X = "he's not a doctor", F="I'm the Pope")
The prices will fall next week, or my name isn't [Name].
If he can hit a ball, then pigs can fly.
If that's pure gold, then the moon is made of cheese.
Not very good examples I'm afraid, but you get the idea.
[1: Entirely unrelated aside: There's an anecdote that once, when Bertrand Russell mentioned in a lecture that starting from a false premise you can prove anything, some smart alec said "1=2, prove you're the Pope". Russell immediately replied with "The Pope and I are two, therefore the Pope and I are one".]
This rhetorical device can of course be entertaining or humorous, as in Dorothy Parker's poem Comment:
Oh, life is a glorious cycle of song,
A medley of extemporanea!
And love is a thing that can never go wrong
And I am Marie of Roumania.
I disagree that the phrase is meant merely to be entertaining. What "if he's an inch" represents is, first of all, an example of the rhetorical device known as prolepsis, which in one of its meanings is the anticipation and addressing of objections to a premise before they may be introduced, in order to weaken opposing arguments.
By saying "She's six feet tall if she's an inch," the speaker is dismissing in advance all arguments that the subject's height may be less than the stated measure. He is in effect summing up the entire list of contradictory assertions ("Is she an inch tall? Is she two inches tall? Is she three ...?" and so on) with the obviously absurd notion that the subject might actually be so ridiculously small, and inviting the listener to laugh at such impossibilities.
For all its rhetorical resonance, however, this is also an example of the logical fallacy known as false dichotomy (similar to a "straw man" argument), in which two things are compared which are not in fact equivalent, so as to dismiss the one that is obviously false and therefore represent the alternative as truth. In actuality both may be false, and therein lies the fallacy. Fallacious arguments sound plausible, and actually constitute a kind of rhetoric (although a spurious one): today's "fair and balanced" news programs use them all the time to sway people who do not look deeply or critically into the "arguments" being made. While the statement "She's six feet tall if she's an inch" may be a bold and emphatic proclamation, it offers no real evidence. It indicates only that the speaker has a firm opinion and wishes to preempt further discussion on the matter.
All these points aside, the "X is [quality] ... if X is [inferior quality]" is a fairly standard and harmless formula in English, and if you want to sound like a native American (assuming you're not) it's one you should feel free to copy.
With regard to the question "Anyone know the origin of this [idiomatic expression]?" statements along the lines of "He's [some estimated size, weight, or age] if he's [one absurdly small unit of measure]" go back to the 1730s at least. The earliest instance that a Google Books search yields involves a comment about a person's age. From Jonathan Swift, A Complete Collection of Genteel and Ingenious Conversation (completed in 1731 but published in 1736, according to Wikipedia):
Lady Answerall. Pray, how old do you take her to be?
Colonel Atwit. Why, about five or six and twenty.
Miss Notable. I swear, she's no chicken ; she's on the wrong side of thirty, if she be a day.
Lady Answerall. Depend upon it, she'll never see five and thirty, and a bit to spare.
The earliest Google Books matches for a wording of this pattern involving size are from the 1820s. From Theodore Hook, Sayings and Doings: A Series of Sketches from Life (1824):
" Dressing-room! cage!" exclaimed Burton ; "why, my dear girl, they [adjutant birds] are fourteen feet high, if they are an inch, as ravenous as tigers, and kick like donkies.
From a letter written in 1827, in The Life and Letters of Maria Edgeworth, volume 1:
Have you heard of the live camelopard, "twelve foot high, if he is an inch, ma'am?" Herschel is well acquainted with him, and was so fortunate as to see the first interview between him and a kangaroo: it stood and gazed for one instant, and the next leaped at once over the camlopard's head, and he and his great friend became hand and glove.
From C.A. Somerset, A Day After the Fair (1829):
Old Fidget. Why Clod, my boy, what ails thee?—What a face you've got ; how did it happen?
Abraham Clod. Why that little crummy French body, as she appeared to be, has knocked me down, for axing her for a buss — and I'm sure I meant no harm by it. (Whimpers.)
Old Fidget. What, that little creature ?—Psha! impossible.
Abraham Clod. Little! egad! she's six foot high if she's an inch.
And the earliest Google Books match involving weight is from 1831. From Sylvanus Swanquill, "A Day at the Trent," in The New Sporting Magazine (July 1831):
And in jumps honest George, heedless of rheumatism and lumbago, thrusts his brawny hands into the fish's gills, and drags him to shore amid the plaudits of his admiring companions. What a beautiful fish! ten pounds, if he's an ounce ; and his fine fat sides covered with blots of silver as large as shillings and sixpences!
From these results, it appears that the phrasing in question has been used in English since at least 1731, but that the form didn't begin to become widespread in writing until the 1820s.
The phrase is perhaps better understood in a prototypical context. Imagine three acquaintances sitting around casually talking:
Man A: So, how tall is this fellow?
Man B: Oh, he's about five feet ten. [inches]
Man C: Nah, he's six feet if he's an inch.
The usage is colloquial and meant to be a more entertaining turn of phrase than a plain statement like "No. I think he is six feet tall." It probably started out being used in contradicting or contrasting situations like above, but it is also found standing alone. It doesn't mean "very tall" but more like "taller than was previously said".