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1.
Someone or something which ruins the ever elusive "fun". A downer or buzzkill.

Someone who is not into what everyone else is doing.
"My roomate is a real fun killer."

"Judy, when you mention your yeast infection when I'm all hot & bothered, it's a real fun killer."

"The cops showing up when we are chucking bottles off the balcony is a real fun killer."
by Leisure Class Hero October 24, 2009
 
2.
A person who comes up with absolutely smart-ass statements to kill off jokes.
Person 1: ...and that's how I got rid of my hangover.

Person 2: That's not true, I work in a dairy factory and it was discovered that milk does not cure a hangover.

Person 3: Goddamnit, Person 2! It was a funny story. *sigh* You stupid Fun Killer
by TRXD February 15, 2012