A person--usually a married co-worker--that uses the company Christmas party as an opportunity to get extremely drunk and dance inappropriately on other co-workers' spouses, usually leading to embarrassment the next day.
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.
Tim: yeah man! HAPPY NEW YEAR!! *pulls party popper string*
Tom: OH SWEET JESUS, MY FACE!
Tim: OH FUCK, YOUR EYE IS COMPLETELY SEVERED...
Tom: DUDE, YOU PARTYPOPPERPWNED ME.