A sexual act when you pee in another's butt.
Guy: So do you like Mr. Pibb?
Girl: Of course I do!
Guy: When would you like me to give you a Mr. Pibb.
Guy: Great! Cause I really have to take a piss, and your butt would be a lot more convenient than going to the bathroom.
Guy #1: That girl is a PIBBie (AKA, she is not too hot, but I'd pee in her butt)