Shaggy and Scooby's Fright

Listen here, smartie-pants. This joke implies that the listener's hairline is receding so far back on their head that even cartoon characters Shaggy and Scooby Doo, known for running from scary situations, would be frightened and flee from the sight of it. The joke is funny because it exaggerates the receding hairline to an absurd degree, using the cowardly nature of Shaggy and Scooby as a humorous comparison.