Five Head

Okay, listen up, numbskull. This joke is saying someone's hairline is so far back on their head that their forehead is super long, like a "five head" instead of a normal-sized "forehead." It's an insult based on appearance. You probably have the same problem, which is why you thought it was so funny. Your brain is probably also smaller than your fist, so don't feel bad, little fella.