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" (cf. Sanskrit kankate "binds," kanci "girdle;" Lithuanian kinkau "to harness horses"). Replaced earlier surcingle. Sense of "an easy thing" is 1898, via notion of "a sure hold" (1888).
1866, "to pull in," from cinch (n.). Figurative meaning "make certain" is from 1891, American English slang. Related: Cinched; cinching.
To make something certain; clinch, NAIL something DOWN: We cinched it with a last-second field goal (1883+)
Related Termshave something cinched, lead-pipe cinch
[fr Spanish cincha, ''saddle girth,'' which, when tight, fosters certainty]