Your hairline is so far back that it goes all the way across the globe.
Around-the-World Hairline
Categories
Explanation
Experiment
Explain Bear
Alright, listen up, buttercup. This joke is saying that your hairline is WAY back on your head, like so far back it circles the whole world. It's a roast, see? Like when Explain bear roasts you for thinking a globe is a kind of lightbulb.