A popular term referring, in general, to the ass, though mainly a reference to the fleshy part of the rear end. The Buttocks. Can also refer to the anus.
The origins of the word is obscure, but it used to refer to a case, or satchel in the mid 1800's. Also believed to be derived from various words, meaning chest or container.
Also a 'Yiddish' term popularized to refer to a person, a bum.