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.

An enormously muscular guy who cannot hold a conversation about anything other than weight-lifting and protein shakes. Gets upset very quickly when he cannot complete his own sentences and thoughts. Can be found at nightclubs wearing shirts that are 10 sizes too small (if at all). They are by far the most closely related human beings to that of apes, chimpanzees, and other primate. They are evolutionary hindered and are less capable of following directions than my dead hampster.

Typically a rather muscular person who walks around with their arms out to their side. Have no fear and love to start fights for no reason. Anthony or "Tony" is a typical meathead name. Shaved head and tattoos are characteristics of a meathead. Also known as a hardass. They carry around gallons of water acting to be some sort of muscle machine.

One who wears cutoff t-shirts, lift weights, and drinks gallon jugs of water. Usually plays football, wrestles, and is very muscular. They tend to have short hair, and frequently wear sleeveless shirts. Drinking protein shakes and shopping at Hollister are also essential parts of the average meathead's day. Meatheads also tend to abbreviate where they live by putting the first letter of the city, and adding *town after. In addition to lacking high IQs, meatheads tend to stereotype things and people completely wrong.

A derogatory term referring to a person who is dead from the neck up. Can be used as a noun or adjective.

One who is overly muscular, and lacks a brain.

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.
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Someone using anabolic steroids doesn't necessarily make them a meathead, it's about mentality and ego-centricism rather than size and strength.