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.
I'm tired of all those idiotic teenage meathead's posting useless definitions on urban dictionary and wasting the time of it's editors.
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."