1. offensive slang Someone with whom to engage in sexual activity; a potential sexual partner. Usually said of a woman. There's a fine-looking piece of ass down at the end of the bar. I'm going to go over and talk to her, see if I can't make something happen.

2. offensive slang By extension, sexual activity in general. Typically said by men to refer to such activity with women. God, I haven't had a piece of ass in weeks! I must be off my game or something.