Filbert can mean the following: Corylus maxima, a species of hazel Filbert, West Virginia a type of artists' paintbrush with an elongated flat bristle with a rounded or circular tip 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").… (More on Filbert)