Is it just another way to poke fun at your friend or is it a literal term? o.O?
Oh I ain't really sure but I do know I sure do hear the term often. I think it is simply another way to poke fun at your friend but I could simply be wrong.
buttmuncher. Noun. (plural buttmunchers) (vulgar, slang) Term of abuse.
Synonym - McFly
Someone that munches on .... Butter! 乂^◡^乂