It is essentially the countious process of MULTIPLE male participants ejeculating or so-called "cumming" into the same female (most commonly the vagina, mouth, or anus). The name comes from the reference to a shack as an often dirty dwelling which can be virtually used by almost anyone (hence the implication of a whore).
Sally is such a hoe! I bet she wouldn't mind becoming the school cum shack!!
Ron: "well barbara, you honestly are the uglyest woman i have ever seen a i know about your pucci cooties , but you have such a way with words. Yes, yes i will shack that for you hunni 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.