One who breaks up or ruins a party through a variety of means. Possible ways of dispersing or destroying a party include but are not limited to: calling the authorities, stories involving octogenarian sexual intercourse, Jennifer Aniston movies.
Rich: Oh no, Larry! Here comes Harold.
Larry: He's probably going to come over here and talk about his wife. She just died. God, what a partypooper.
Rich: What a selfish bastard! What makes him think we care about that?