A meat head is someone who tries really hard to be ultra-masculine. This involves lifting weights, talking about lifting weights, walking with your arms out because you're so huge, looking to get in fights at bars because the ladies aren't down with you or because someone ran into one of your arms or just because you saw an emo dude with a stupid shirt. Meat heads always wear Affliction shirts or vests.
Contrary to popular opinion, being a meathead is more about how you act than how you look. Therefore, a skinny dude or even a bull dike could be a meathead, cuz they exhibit this behavior.
Look at that dude over there by the ladies restroom with the cutoff Affliction shirt, what a meat head.