Roast
You have such a big forehead it has a 6 pack on it!
Six-Pack Forehead
Explanation
Explain Bear
Listen up, dummy! This "joke" is saying your forehead is so huge, it's got a six-pack of abs on it. Get it? Like, it's bulging out so much it looks like muscles. You probably don't even know what a six-pack is, huh?