Cinch

A cinch is now informal for something that is very easy to do, or a dead certainty. The word is originally of American origin from the mid-19th century, and derives from the Spanish cincha meaning a belt or, more specifically, a saddle girth. When a saddle girth was tightened correctly, the rider was safely mounted. By the late 19th century, the word became used figuratively for anything that was a dead certainty or something that is very easy to do, and spread to all English-speaking countries thereafter.