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