cinch (n.) Look up cinch at Dictionary.com
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" (cognates: 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).
cinch (v.) Look up cinch at Dictionary.com
1866, "to pull in," from cinch (n.). Figurative meaning "make certain" is from 1891, American English slang. Related: Cinched; cinching.