Green’s Dictionary of Slang

cinch n.1

also sinch, skinch
[SE cinch, to grip tightly; thus something one can grasp easily. Orig. f. Sp. cincha, a saddle girth or bellyband, adopted in US Western use]

1. [late 19C–1900s] (US) a fool, one who can be easily overcome [ext. of sense 2].

2. [late 19C+] (orig. US, also kinch, sinkers) a simple, easily attained thing, a certainty; thus cinchy, sinchy, easy, easily attained or attainable; cinch-looking, apparently easy.

3. [1930s] (US) an easily seduced woman.

In phrases

have the cinch on (v.)

[late 19C–1920s] to place oneself in an unassailable position.

put the cinch on (v.)

[late 19C–1960s] (US) to ensure, to make something certain.