A person who acts stupid but is reallysmart 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.
12 step recovery jargon referring to someone new who talks about how great life is, now that they'resober. Usually meaning that the person is out of touch with reality.
The new guy seems pretty happy for a dude who has no job, no money and no family. He must be on a pink cloud.