Meaning

noun a dull stupid fatuous person

originFirst use appears in the US c. 1961. The sense of a "silly person" is presumably from earlier use as a bowdlerization of dick (“penis”) in student slang, particularly the Midwest. Despite a common misconception, it has never referred specifically to a whale's penis, but penises in general. Alternative etymology derives from dialectal Norwegian dorg (“a mass; heap; a heavy, dimwitted, slovenly…

Etymology from Wiktionary, CC BY-SA 3.0