Green’s Dictionary of Slang

1. (US) to impose upon.

a.1889

in Barrère & Leland Dict. of Sl., Jargon and Cant I 252/2: My father is wealthy, and I think I can cinch him for five hundred dollars.

2. (US) to defeat, to overcome, to trounce.

1875		Scribner’s Monthly X 277/1: A man who is hurt in a mining transaction is ‘cinched’ [DA].
1881		N.Y. Times 18 Dec. quoted in N&Q Ser. 6 V 65: Cinch. To subdue, to forcibly bind down and overcome. Thus it is unfairly said that the Northern Pacific Company intends to cinch the settlers by exacting large prices for its lands. Query, from Latin cingere [F&H].
1903		Dly Pioneer (Bemidji, MN) 8 May 4/2: Chinches are Cinched [...] the pest of chinch bugs is at an end now.
1910		T. Roosevelt Outlook 3 Sept. 2/1: If the rich man strives to use his wealth to destroy others, I will cinch him if I can [DA].
1917		F. Packard Adventures of Jimmie Dale (1918) I xi: Tell your story—who’d believe it! I got you cinched.
1935		H.L. Davis Honey in Horn 231: It’s about ten to one they’d cinch you.
1940		O. Strange Sudden Takes the Trail 177: He certainly has Kent cinched.
1967–8		Baker et al. CUSS 97: Cinch Do well on an exam.

3. (orig. US) to guarantee, to make certain, to make conclusive.

1895		Crowe Aus. Sl. Dict. 17: Cinch, to get the grip on; to corner, etc.
1900		Nat. Police Gaz. (NY) 24 Mar. 6/1: [headline] How Two Clever Crooks / [...] / Beat A Roulette Wheel / [...] / They had The Wheel Cinched With Fixed Numbers.
1903		A.H. Lewis Boss 315: He goes after him to cinch th’ play.
1906		S. Ford Shorty McCabe 192: He means to go back [...] with the little matter of the two million ready all cinched.
1914		R. Lardner You Know Me Al (1984) 38: So I guess my job is cinched.
1927		(con. 1900s) S. Lewis Elmer Gantry 99: We’re here to do good, but first you have to cinch a job that you can do good in!
1933		B. Ross Tragedy of Z 67: Yep, that cinches it.
1946		Mezzrow & Wolfe Really the Blues 69: I’m gonna run a high-class club, and your band would cinch it in that town.
1952		J. Jones From Here to Eternity (1998) 336: Figuring to milk the last drop out of him, figuring he had this one cinched.
1963		F. Kohner Affairs of Gidget 50: I thought it cinched on account of my past experience, but the manager turned me down.
1972		D. Jenkins Semi-Tough 66: We’re undefeated and untied and already have a diamond ring cinched for winning the National Conference.
2006		sports.yahoo.com 15 Jan. 🌐 He sprinted down the sideline and when he felt Kevin Faulk swipe at him helplessly [...] he thought he had it cinched.