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1.
An depressing person. One whose presence tends to darken others' moods, consistently.
We were having a great time, until the buzzkiller arrived and started whining about his dating troubles.
by tlc August 27, 2003
 
2.
Some one who kills your mother fucking good time.
For example, the cops, your parents.

Man that cop was a really buzz killer!
by RockinThatShit June 30, 2008
 
3.
Someone who fucks up when you are having fun.
Damn it Pranjal, You are such a buzz killer!
by GodFreak July 27, 2011