A person whose mother was a slut. The term dickhead is used because when the baby was in their mother and she was having sex, dicks would poke the baby in the head. Dickheads tend to have butt chins because the babies soft jawbone was getting poked by dicks which in turn left an indent.
Hey bro you better not fuck that girl, you don't want your son to be a dickhead.
Damn! Dicks must have been all up in that baby's face. Poor kids a dickhead.