Filbert can mean the following: Corylus maxima, a species of hazel nut Filbert, South Carolina Filbert, West Virginia Filburt, a character of the TV show Rocko's Modern Life Slang: a lunatic; an individual wholly unhinged; predominant neurotic tendencies (for example, "a nut"). Filbert paintbrush, a type of artist's brush that's an ovoid tip which resembles a hazelnut… (More on Filbert)