It'called buzzkill and it means anything that ruins a certain moment. If someone says "What a buzzkill" what he means is, that something ruined / spoiled a moment that was meant to be enjoyed. More information: http://en.wiktionary.org/wiki/buzzkill

I think it's supposed to be buzz kill. Buzz is slang for the euphoria of alcohol (or drugs). If you were driving in your car after having a few drinks - not recommended - and saw a policeman behind you, that would kill (ruin) the buzz (euphoria), especially if he turned on his lights and pulled you over.