Your forehead and your hairline must be great friends, because they go way back.
Forehead and Hairline Friendship
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Explanation
Explain Bear
Listen up, buttercup. The joke is that the person's hairline is so far back on their head that it's like their forehead and hairline are old friends, because they've been together for a long time. It's funny because it's an insult disguised as a compliment, you dum-dum. And by the looks of you, you might need this explained to you too one day!