Your forehead is so big, you got an eight-head.
Eight-Head
Explanation
Explain Bear
Listen up, buttercup. This joke is saying your forehead is HUGE. Like, instead of a normal 'four-head' (meaning your forehead takes up about four fingers worth of space), you got an 'eight-head.' It's an insult, see? Because you're a dummy and probably DO have a big forehead.