Cinch, derived from the Spanish word cincha ("saddle girth"), came to mean a firm hold or a tight grip because of the physical object used in securing a horse saddle. It then became an American idiom for something sure and's a cinch!

Cinch is also used as a verb, both meaning "to fix a saddle securely" and in more widespread usage, "to make secure; to tighten" (for example, if I were to "cinch my belt" I'd be a happy camper because it probably meant I lost weight and could now tighten my belt further *grin*). thanks, anotherone