A person who (in the eyes of his wider social group) is either unenthusiastic or not committed for nights out or events, and therefore lowers and endangers the social atmosphere.
A: Everyone's going out tonight, its going to be mental! Are you coming?
B: Unfortunately, I've got some work in for next week. Plus, I don't really have much money at the moment...
A: Mate, Your such a kimmer.