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.
[late 19C–1920s] to place oneself in an unassailable position.
[late 19C–1960s] (US) to ensure, to make something certain.