Cinch

A cinch: An absolute certainty. Something easy to do, e.g., "It's a dead cinch." Apparently derived from the Mexican word "Cincha," a belt or girdle, a saddle girth. The "C" is pronounced soft, as in "cinders" in Mexico and the American Southern States. In Canada and the Northern States it is pronounced hard as "K".

To cinch: To grip or fasten.