Someone with a head the size of a watermelon on top of their neck. Usually they will claim that it is their hair that makes their head so big they can not even wear a standard hat, unless it was made for an elephant, but in the end their skull really his at minimum the size of a bowling ball, and usually their brain is about the size of a pea. Even though their head is unusually large, they are quite retarded and often times have what is known as "Fat-Head Syndrome". That is where they think they know everything, and have advice on things they have never even done before. Fat-Head Syndrome also gives said fat headed person an ego the size of their dome, which makes them think they are a bad-ass. Most commonly a fat headed person will work out all the time so they can try to grow their small bodies to match their fat ass head.
Hey Fat Head, why is your head so damn big?
- Its not my head, it is my hair
Oh, okay, how come you have to wear a neck brace to support that massive melon?
- I don't. I am going to the gym to work out.
Oh, well hopefully you can grow into that head of yours..... Maybe try to get a brain to fit that noggin too.