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)