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A person who acts stupid but is really smart but only when it is needed.
Lets say Bob goes to school and steals some cafeteria food, pretty stupid. But when he takes his Algebra final he makes a 100. This would be a jawmuncher.

This word is commonly used in middle and high school when talking about a friend that does stupid things outside of school but is smart inside of school.
by Max June 16, 2006
A stupid person who turns out being smart.
That dude is a jawmuncher right there.
by John Scadlock June 15, 2006
An idiot, or otherwise unintelligent person. A pratt.
He's a Jawmuncher
by Anthony Hawk August 26, 2008