Combination stank of swamp nuts/twat, unwashed/wiped ass, armpit, dirty socks, road grime and bad breath producing a funk akin to a bag of rotten potatos. Usually associated with truck drivers that have been on the road for extended periods without a shower.
Shop Planner: what the hell is that nasty funky ass smell?
Parts Coordinator: oh, that was potato butt! that driver never takes a shower!
It is said of the situation where a person has the bad luck to make contact with his testicles against an undefined surface or object, intentioned or not.
Given the nature of the word, it is more appropriate to design cases where the interaction is made with a moving object, for example, a ball.
Although it is extremely painful for the victim, it tends to be considerably funny to people who witness it.
Today in the baseball game the pitcher took a nutshot; the baseball hit him in the nuts.
Man, I just watched the funniest nutshot video ever.