Green’s Dictionary of Slang

duh! excl.

[echoic]

a grunt of incomprehension; often used as a rejoinder, implying that the first speaker is stupid; often in positive or negative phrs. well, duh! or no, duh!

[US](con. 1950s) H. Junker ‘The Fifties’ in Eisen Age of Rock 2 (1970) 100: Duhhhh! You’re so dumb, you think manual labor is President of Cuba.
[US]Eble Campus Sl. Fall 3: duh, duh, duh – etc. Mark was telling me that he got his shirt from Atlanta and his pants from this store and duh, duh, duh.
[Aus]T. Winton Lockie Leonard, Legend (1998) 187: ‘There’s noone around,’ said Lockie [...] ‘Only the police, Lockie,’ said Phillip. ‘Like, duh!’.
Online Sl. Dict. [Internet] no-duh 1. a sarcastic response used when someone states the obvious. (Statement: ‘Don’t run that red light.’ Response: ‘No-duh.’ Statement: ‘I bet it hurt when you got shot in the chest.’ Response: ‘No-duh.’).
[Ire]P. Howard Curious Incident of the Dog in the Nightdress 213: That’s like, hello? No, actually, it’s more like Duuuhhh!
[SA]Constitutionally Speaking (S. Afr.) 28 Oct. [Internet] As fvor Zapiro — a double ‘duh!’ is in order’.