The guy that purposely stands in the way in a bar or club waiting for someone to make eye contact or brush past him so he can start a fight.
A meathead will spend all his time lifting weights and generally being an arrogant prick whose goal primitively is based on intimidating other males who're often a lot smaller than them.
The mentality of a meathead is that having the capability to beat up the majority of normal sized males makes him better than everyone else and makes up for his poor intelligence.
A meathead will often make cruel/physical jokes to friends and acquaintances, meant to appear light hearted, but actually are just assertions of their power- which they'll follow by "i'm just pulling your leg" or "I was joking mate! Don't look so serious!".
Avoid these guys when they're out on the piss because they take the opportunity and pleasure to fight ANYONE. Can be identified by bulging muscles, tight tops and many girls under the age of twenty asking to buy them drinks.
Someone using anabolic steroids doesn't necessarily make them a meathead, it's about mentality and ego-centricism rather than size and strength.
Friend 1: "Looks like another meathead is kicking off and spoiling everyones night again."
Friend 2: "Hopefully he's barred and we don't have to look a this dick week in week out."