A boy's hairline is always in the back of his head, and its shape is like a check mark.
Check Mark Hairline
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Explanation
Experiment
Explain Bear
Listen here, smarty pants. This joke is saying the dude's hairline is so far back on his head that it looks like the Nike swoosh. It's funny because it paints a picture of mega-receding hairline. I bet you don't even know how to spell 'receding'.