Other contributors have said "Is it buck naked or butt naked?" is the same question as "Is it butt naked or buck naked?". If you believe that these are not asking the same thing and should be answered differently, click here.
Is it butt naked or buck naked?
Answer:
"It is both. The word "buck" or "buff" is thought to be from the color of a buckskin, which is the pale tan color of European skin - this gives you "buck naked" and "in the buff." "Butt naked" refers to the fact that your buttocks are not covered."
I have always heard it "buck naked". I don't think it refers to any color at all; the phrase was originally meant to compare one to an "Indian buck" as the men were called many years ago in a less politically correct age. They were commonly thought to be "naked savages", whether they were in fact or not. Thus, "buck naked" implied being without clothing.