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Pre-munch is essentially food that has already been munched but accidently leftover from a previous person, is then munched again by another person.

Food that has been chewed, but leftover. IE. the act of previously being munched on food. Food that has literally just about to be eaten and has freshly been chewed on.
Greg was eating a carrot while drunk, left some dangling of a pre-munched carrot. Jenna then proceeded to grab that carrot for a bite only to find the pre-munch crumbling to bits before taking a bite. "Dude, I just totally ate your pre-munched carrot..."
by CR Representitive February 16, 2012