this has been discussed many times (even here), and I bring it up again just because I've recently seen two examples (proving that it's an ongoing dichotomy, I guess):

"Guy stands up buck naked and takes a little bow."
- Michael Connelly, Trunk Music (1998)

"Armstrong is pushing himself so hard on this Tour that if you want to see him, you have to see all of him, butt naked, on the massage table."
- Rick Reilly, ESPN July 9, 2009

I always thought "buck" was the original, and that "butt", which obviously sounds similar, seems to make more sense, and so began the split. I guess I don't really know what "buck naked" is supposed to refer to. A deer? If so, why? Aren't all animals pretty much naked all the time? And some are naked with just a shirt, or just gloves, or a tie or whatever hunters, apparently, leave in the forest... ;0)

Certainly buck naked would lead to butt naked by assimilation. Still, I would like to see some evidence of one form preceding the other by some significant amount of time. Claqssic Language Log addressed the subject some time ago with no definitive answer.

Never heard of "butt naked" before. Always only heard the term as "buck naked". One theory proposes the original phrase was actually "butt naked." The phrase was then cleaned up to "buck naked" so it could be used in polite company. The word "buck" in this sense is an adverb meaning "stark" or "completely."

research? did someone mention research??

bare-naked: 1,190,000 google hits
buck-naked: 358,000 gh
butt-naked: 1,210,000 gh

I am astonished by these numbers. I am flabbergasted that AHD4 suggests that butt-naked came before buck-naked. I am gobsmacked by Urban Dictionary's no. 1 def'n: Americanised version of buck naked. Probably arisen from the Yank inability to speak English.

more to come

-joe (wdf?) friday

language log weighs in.

edit: most of the AHD4 links out there are dead, as they refer to the Bartleby artifacts. here is a viable one: yahoo! link [funnily (or not), if you OneLook butt*naked you get very few hits.]

you could always take the quakebuttock's approach and used stark naked.

Which has, of course, been eggcorned to start-naked. (Or was it the other way around?)

eggcorned to start-naked. (Or was it the other way around?)

Why is it that the idea of an unknown or uncertain etymology is about as burr-under-the-saddle-itch-producing to the amateur lexicographer, as a language isolate is to the derive-all-languages-from-my-super-duper-proto-language comparative-historical linguists.

>Why is it that the idea of an unknown or uncertain etymology is... burr-under-the-saddle-itch-producing to the amateur lexicographer

I have no idea, except that "unknown origin" (or words to that effect) encourages you to attempt something - or acts as a goad, more like.
-joe (the unknown amateur) friday

encourages you to attempt something - or acts as a goad, more like.

It was a rhetorical kvetsh.