Green’s Dictionary of Slang

1. (US) a fool, one who can be easily overcome [ext. of sense 2].

1894		S. Crane in N.Y. Press 9 Dec. in Stallman (1966) 113: Say, if yeh pick me up fer a cinch, I’ll fool yeh.
1902		Boston Globe Sun. Mag. 21 Dec. 7–8: A professor who is at all lenient in his methods of instruction is a ‘cinch’.
1906		Eve. Star (Wash., DC) 29 July 34/1: I’m a cinch [...] Every panhandler [...] knows he can work me for a beer check.
1919		D. Runyon ‘A Tale of two Fists’ XV in Pittsburgh Press (PA) 16 May 31/1: ‘I used to be cinch for any one to pop on the chin’.

2. (orig. US, also kinch, sinkers) a simple, easily attained thing, a certainty; thus cinchy, sinchy, easy, easily attained or attainable; cinch-looking, apparently easy.

1887		Lantern (N.O.) 5 Mar. 3: Rube claims to be a tout, and gets around the greenies saying he has a sinch but if he ain’t careful, he’ll bite the dust.
1895		E. Townsend Chimmie Fadden Explains 25: I mean dose pieces what says what mug has de dead cinch on bein de dandiest dresser on de avenoo.
1899–1900		C.L. Cullen Tales of the Ex-Tanks 223: The cinch-looking price of 3 to 5 or 1 to 2.
1901		‘Hugh McHugh’ Down the Line 16: ‘A skinch!’ said Nash. ‘Go as far as you like.’.
1903		A.H. Lewis Boss 96: He grafts nights [...] an’ at this time of day it’s a cinch he’s takin’ a snooze.
1903		A. Kleberg Slang Fables from Afar 12: [T]the winning of the hearts of the Kroywen Fancy Work was Sinchy for Browne.
1904		St Paul Globe (MN) 7 Aug. 27/2: It’s sinkers fer de bunch dat Romeo and his bundle is up de pike in a minute on the merry get-away.
1906		Sun. Times (Perth) 1 July 4/7: From him to sneak to a job with Leake / Was a ‘kinch’ to my acrobat organs.
1912		H.G. van Campen ‘Our Theatrical Boarding House’ in L.A. Herald 10 Dec. 10/4: ‘Don’t yuh never git the notion bein’ a mechanic’s a cinch, sister‘.
1914		M. Glass Potash and Perlmutter 1: This brokering business ain’t such a cinch neither.
1919		Wodehouse ‘Leave It to Jeeves’ in My Man Jeeves [ebook] ‘It’s a cert!’ I said. ‘An absolute cinch!’ said Corky.
1921		N. Putnam West Broadway 168: ‘Believe me, crossing this man's country is no hardship, but it's no cinch, neither!’.
1925		Wodehouse Carry on, Jeeves 35: ‘It’s a cert!’ I said. ‘An absolute cinch!’ said Corky.
1929		W.R. Burnett Little Caesar 37: This ain’t going to be no cinch.
1931		Ade Old-Time Saloon 138: The syndicate [...] appeared to be making a safe bet, because [...] it seemed certain that the profits would continue and the dividends would be a cinch.
1936		Truth (Brisbane) 2 Feb. 42/1: ‘It’s a cinch, you'd have been late for breaker’.
1937		J. Curtis You’re in the Racket, Too 61: It’s a cinch, we’ll have it off sweet as a nut.
1940		Wodehouse Eggs, Beans & Crumpets ((1951)) 112: [W]hen he had conceived the scheme of using Algernon Aubrey as a softening influence, he had felt that it was a cinch.
1946		Mezzrow & Wolfe Really the Blues 53: Why didn’t I ask my sister Helen to take down the words in shorthand? She was doing secretarial work and [...] it would be a cinch for her.
1953		I. Hamilton Till Human Voices Wake Us 5: They conquered the Maoris and they cut down the bush and it looked a cinch.
1953		Kramer & Karr Teen-Age Gangs 9: Cinch handled a few stick-ups and was, as he said, a cinch at stealing cars.
1969		‘Iceberg Slim’ Mama Black Widow 71: It’s a cinch I ain’t gonna’ burn my own baby brother.
1978		P. Theroux Picture Palace 43: It is not the cinch it seems for a photographer to take her own picture.
1989		C. Hiaasen Skin Tight 40: A cinch [...] We can do it under local anesthetic, make it smooth as silk.
1999		E. Bunker Mr Blue 215: Within the month he was going to court for sentence and it was a cinch he was going where I had just been.
1999		Guardian Editor 25 June 16: It’s a cinch. You just chuck the eggs and flour together and knead for glory.

3. (US) an easily seduced woman.

1932		J.L. Kuethe ‘Johns Hopkins Jargon’ in AS VII:5 330: cinch—[...] a woman who is easily possessed.
1952		F. Weismantel ‘Big Ten-inch Record’ 🎵 Got me the strangest woman / Believe me this chick’s no cinch / But I really got her goin’ / When I take out my big ten-inch . . . / Record.