Green’s Dictionary of Slang

(US) definite, guaranteed.

1890		J.P. Quinn Fools of Fortune 562: The bookmaker’s agent asumes the role of a gentleman who [...] has secured a ‘cinch tip’ (meaning a sure thing).
1906		S. Ford Shorty McCabe 251: Talk about your cinch jobs! those are the lads that can pull ’em out.
1908		B. Fisher A. Mutt in Blackbeard Compilation (1977) 54: If you want a cinch bet put a little chunk on Britt.
1914		R. Lardner ‘Horseshoes’ in Coll. Short Stories (1941) 253: It looked like a cinch double play.
1924		H.L. Wilson Professor How Could You! 319: It’s a cinch bet they are now chasing me to K.C.
1944		Democrat 14 Aug. 15/3: GI’s ‘Cinch Bet’ was Newsweek Boner [DA].
1953		W. Eyster Far from the Customary Skies 111: Oklahoma bet two chips on his pair of tens showing. No sense scarin’ ’em out on a cinch hand.
1969		‘Iceberg Slim’ Pimp 221: As soon as I read they was found in an alley with their skulls caved in he could get a cinch two grand.
1974		(con. 1940s) E. Thompson Tattoo (1977) 237: She’ll go, man. You gotcherself a cinch piece of tail there, boy.