a stained anus, kinda browner than you think it should be...maybeeven a dingle-berry or two present. Rhymes with anus.
Biff: Hey Blaine...that chick I banged last was awesome. But, when she bent over and pulled her panties off, I saw her stainus! It was rotten. But, I did her anyways.
Blaine: Did your tool smell like ass?
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.