Green’s Dictionary of Slang

a fool, an irritating person; a conventional person.

1960		C. Cooper Jr Scene (1996) 83: You inter-rupted my conversation, stiffy!
1965		W. King ‘The Game’ in King Black Short Story Anthol. (1972) 305: You a trick Dan – a stiffy.
1966		C. Keil Urban Blues 117: [note 6: The difference between white and black culture can [. . .] be summarized concisely as the difference between white march music, in which the accents fall on 1 and 3 of a 4—4 measure, and Negro dance music, which emphasizes 2 and 4, the ‘off beats’ or ‘back beats.’ Some Negro artists who find their way into white concert halls still find it necessary to ‘hip’ those ‘stiffies’ in the audience who insist on clapping their hands in a martial manner.