Green’s Dictionary of Slang

not likely! definitely not!

1900		Marvel XIV 15 June 343: Bill’s smiling apparatus wasn’t in very good working order since he got wounded. Nunno!
1908		H. Green Maison De Shine 214: Nunno! [...] William, I forbid you to listen!
1918		Butterfly and Firefly 23 Nov. 8: But Percy wasn’t going to stand that. Nunno!
1920		Marvel 1 May 2: ‘Were you trying to teach the Chinese image to dance?’ ‘Nunno, Algy!’ growled Pete.
1944		J. Cary Horse’s Mouth (1948) 11: ‘Are you going to school yet?’ [...] ‘Nunno,’ he said.