Uncle Dave is the uncle of Shaggy Rogers.
Apparently, Uncle Dave has been critical about Shaggy's masculinity.
Be Cool, Scooby-Doo!
During Fred Jones and Daphne Blake's debate on why the latter was wearing a beard, she argued that Shaggy had a beard (technically a goatee), but Fred countered that, that was because he was a man. This proved a victory for Shaggy, over what his uncle thought about him.