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
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."