When the person your
talking to is making you too mad to think so after say your closing line you deliver a powerful punch to the cunt and walk away.
The term also has roots from the term
punch line due to the fact that they both end conversations on high notes for
the speaker.
Bryan: Do we shower after PE?
Jacob:
I don't know, we'll see.
Chris: Huh, Jacob, I hope my size doesn't affect our
friendship.
Jacob: No dude, I won't laugh at you.
*Chris loses
douchebag grin and just before he tries to say some gay remark Jacob delivers a devistating blow to the cunt hence the term cunch line*