because the shape of there head looks like an egg.
matt: f*ck off
Harry: matt you egghead , you should of gone to spec-savers
Teacher: whats wrong matt you like abit..
scrambled , are they crackin' jokes towards you again.
2. A person's whose head is shaped like an egg.
Most people however, will use this word interchangeably as a pun. It has also been known that people whose heads are shaped like an egg are usually large at the top, which explains the larger brain-size.
James: *awkwardly stares at Rick*
Rick: Uh, hello James are you gonna respond?
James: *Still is on complete silence*
Rick: Oh that's right, James is an Egg head. Oops