Buttmunch is a common insult used to describe someone with a disagreeable personality. Character traits that can define a person worthy of being called a buttmunch include but are not limited to: annoying, mean, dumb, sarcastic, etc.
No ones knows were the term was originally coined before it was used in Beavis and Butthead, which brought the word to mass popularity.
Haley: Blah Blah *annoying* blah blah blah
Daniel: Haley! Stop being such a buttmunch!!!!