Your hairline goes so far back that the History Channel made a show about it.
History Channel Hairline
Categories
Explanation
Experiment
Explain Bear
Okay, listen up, smarty pants. This joke is saying your hairline is receding so much that it's like ancient history. Like, the History Channel would make a show about it because it's that old. Get it? You probably don't, because you're probably balding too, judging by your IQ.