Receding Hairline as McDonald's Logo

Alright, listen up, you big dummy! You know that big, golden 'M' shape of the McDonald's logo? It's basically two big arches, right? Well, when someone's hairline is really going backwards, it can start to look like those same two arches! So, the joke is saying your hairline is so far gone, it's basically a walking McDonald's sign. You're probably scratching your head trying to figure out if you're hungry for fries now, aren't you?