But hold on a second. I know a lot. Or at least, my auxiliary brain does. Let's check Google!
First check: "wild hair" + "ass" or "butt" returns 78,000 hits. "Wild hare" returns only 25,000 hits.
There are two expressions, wild hare and wild hair. The first refers to or compares someone or something to the natural skittishness of breeding hares in spring, especially in March (ergo Lewis Carroll's inclusion of that creature in the Mad Hatter's tea party). To have a wild hair (up one's butt) is a vulgar expression indicating an obsession or fixation of some sort. "Wild" in the first instance denotes erratic behavior like that of hares in rut. In the second instance "wild" characterizes a stray or unruly strand whose indelicate lodgment is the figurative cause of someone's perceived mania.Disagreeing, in a typical Usnet digression from a discussion of Python programming minutiae, is James Stroud:
I think most Americans say "wild hare up your ass". We do not, in fact, say "wild hair up your ass". Many of us can testify that a hair up one's ass would be nothing terribly unusual and would go completely unnoticed under most circumstances.And finally, voting for "wild hair," here is word maven Doug Wilson:
[One day later:] We say "wild hare" down in Texas. I think I've heard "bug" before, but I wanted an excuse to vent about the hair v. hare issue in some of these American idioms. I guess I have a <insert idiom here> about it.
The mystery here (at least to me) is how this expression came to be. Lighter gives examples only since the 1950's, but "A Wild Hare" was the title of one of the earliest Bugs Bunny cartoons, 1940 I think, and I'm sure it was a play on the above expression or at least on some conventional expression of that time. Sometimes it is said that the "wild hair" in the rude expression is an ingrown inflamed perianal hair, but this seems retrospective and bogus to me. There is/was an expression "get hared up" meaning something like "get startled" and I wonder whether this mutated into "get a hair up" which was then augmented and clarified in a rude fashion (or maybe it went the other way!). Maybe there also was once a conventional metaphor like "wild hare" = "irresponsible person" or so? Or maybe "a wild hare" = "a wild idea" [for some reason] or even "a wild run"?Later in the same thread, Rick Kennerly presents the case for the phrase's origin in "wild hare," with references back to Chaucer and Erasmus. Other hints: The New Partridge Dictionary of Slang and Unconventional English lists only "wild hair." As Wilson notes above, the first Bugs Bunny cartoon, "A Wild Hare," came out in 1940. The Wikipedia entry, with no source cited, says "The title is a play on 'wild hair.'" Reference.com has no entry for either term. The readers of a Chronicle of Higher Education forum voted 45% to 17% for "wild hair." Cheating a bit and going to my actual physical reference shelf, none of my four slang dictionaries has a listing for either phrase.
That's it. For now, I'm sticking with "wild hair." But it is remarkable what a fantastic timewaster Google can be, isn't it?