partial mirth abortion 

The act of cutting off someone's joke by beating them to the punchline.

A: Hey, did you tell your co-workers that joke I told you last night?
B: Yeah.
A: Well, how did it go over?
B: It was a 'partial mirth abortion'. I was pausing, as you should for comedic effect... just about ready to tell the punchline..... and some jerk beat me to it.

partial mirth abortion by Bryan Gilbreath March 1, 2011