The phrase buck naked is well known and means "completely naked". It is synonymous to butt naked and stark naked, both self-explanatory. However, there are a few confusing aspects to the etymology of buck naked.
My research brought me across a few plausible explanations:
The word "buck" comes from buckskin, which is a deer/horse like animal.
A slang used by Americans against natives(Indians) for young males who often fought or hunted without clothing.
It is a Bowlderization of "butt naked" so that it can be used in public without causing unnecessary discomfort. An euphemism of some sort.
All three explanations seemed equally possible to me and I failed to unearth an authoritative source to confirm any of them.
It would also be interesting to know if the phrase is more popular in some parts of the world than others.