Duh is an exclamation denoting
room-temperature IQ. It originated in Archie comics, in which there was a stereotypically big, blond, dumb, athlete named Moose. It was said that his brain was in his big toe, implying that the only thing he was good for was kicking a football.
Everything (and I mean everything) that came out of his mouth was prefaced with "Duh!" which demonstrated that he was a complete dolt.