cinch n.

1859, American English, "saddle-girth," from Spanish cincha "girdle," from Latin cingulum "a girdle, a swordbelt," from cingere "to surround, encircle," from PIE root *kenk- (1) "to gird, encircle" (source also of Sanskrit kankate "binds," kanci "girdle;" Lithuanian  kinkau, kinkyti "to harness horses"). Replaced earlier surcingle. Sense of "an easy thing" is 1895 (in lead-pipe cinch), via notion of "a firm or sure hold" (1888).

1866, "to pull in, gird with or as with a cinch," from cinch (n.). Figurative meaning "make certain" is from 1891, American English slang, via Western U.S. colloquial sense "bind or subdue by force" (1875). Related: Cinched; cinching.