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.
a person who is not jewish, but hangs out and surrounds themselves with jews so they might as well be. and they also probablymunch on jew dick/puss , hence the muncher part.