A meathead can be anyone of combo of the following things:
1. Someone who is muscular and slower in the brain.
2. Someone who marinates their head in sauce.
3. Someone who likes to put their heads in a grill of some kind (Ex. Barbeque, George Forman Grill, ect.)
4. Someone who is a complete fucking idiot.
A: Yo look at that guy, he's jacked!
B: Ya, but he's a fucking meathead though.
A: Why is that idiot grilling his own head?
B: Probably cause he's a fucking meathead.
Someone who posts their name on Urban Dictionary followed by a bunch of untruthful egotistical comments. Usually done by cowardly teenage jocks with undersized genitalia.
A male who drives a lifted truck, wears Tapout clothing, follows UFC, wears shades and a hat (sometimes backwards), listens to metal, has a submissive, blonde girlfriend. Some are muscular, others a twigs who act tough.
Passenger: "Who was that guy tail-gating you?"
Me: "Probably some meathead."
Passenger: "How do you know?"
Me: "It was a lifted truck with a Tapout bumper sticker."