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1.
The act of saying something absurd when someone enters a room so as to make them think that you're discussing something very bizarre.
When Pat came into the room, Joe engaged in punchlining him when he said out of nowhere "and that's the last time I ever set a penguin on fire in my bathroom!".
by Mr_Shifty November 12, 2008
 
2.
The act of repeating the funniest part of a joke or movie quote to your friends after they have all laughed at it, causing more laughter among the group.
Jeff: "dude that joke Chad told was hilarious."
Greg: "oh yea we were punch lining that one all night, it never gets old."
by Parkersaur June 12, 2011