Originates from Smodcast (Kevin Smith and Scott Mosier podcast) Episode 91 at the 52:44 mark where guest Malcolm Ingram credits uber-producer Scott Mosier with the genesis of the term.
This is a word describing someone who kisses up to girls/ tries to gain their attention hoping they'll get lucky.
Why it is "nutmuncher" instead of "vaginamuncher" I have no idea. It's just the way things are.