Your hairline's so far back, you need binoculars to see it.
Man, your hairline is so far back, archaeologists couldn't find it.
Okay, so listen up, you smooth-brained ape. This joke is all about how someone's hairline is receding, like, a LOT. They're saying it's so far back that you need binoculars to see it, and even archaeologists can't find it. It's an insult, see, because a receding hairline can make someone look older than dirt. It's an insult because you have the same hairline.