Will Smith Slap

Listen up, you silly goose! This joke is saying the person's hairline is receding SUPER far back. So far that even when Will Smith (a famous actor) slapped it (likely referring to the infamous Oscars slap), it's still too far back to see. That's a wild reach. BTW, fun fact, about 85% of men will experience some form of hair loss in their lifetime. Don't let it get you down, buttercup!