This is a very rude name for a person you think is stupid, or who has done something you hate. Do not use this word. It follows an old insult pattern: stick a dirty word in front of head, and the whole thing means “stupid, nasty person,” the same shape as dickhead. You will hear it when someone is already furious, not in careful talk. To many ears it sounds more American than fuckwit. There is no polite version, and no place at school or work where it is all right. People are not talking about a body part; they are just swearing at someone’s brains.