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).
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"