No ones knows were the term was originally coined before it was used in Beavis and Butthead, which brought the word to mass popularity.
Daniel: Haley! Stop being such a buttmunch!!!!
1. An annoying person that you would like to shut up.
2. A bothersome sibling.
3. A friend who is being a jackass.
Person 2: Shut up you freaking butt munch.
Syn.: jerk, wanker, bastard, fucker, creep, buttmonkey, asshole, shithead
"All you do is make fun of me. You're such a buttmunch."