This from http://makeashorterlink.com/?F26941C8
(not sure of the credibility of the info but sounds okay - it's from a site that teaches english on the net).

knucklehead

Definition: A stupid person.

Example: 1) Charlie's a real knucklehead -- he's been in first grade for five years! 2) You knucklehead! How could you have mixed the the dog food with the cereal?

Etymology: A 'knuckle' is a joint or a place where bones meet, particularly the bones of the hand. This word suggests that someone's head looks like or has been hit by a hand, thus making the person stupid. And for whatever reason, the phrase just sounds funny.

That's the best I could come up with DubDub...

Hev