Something that's a cinch is incredibly easy. It's a cinch to eat a meticulously decorated cake — it's much more complicated to bake and frost one.
Cinch is one of those words with many meanings that seem unrelated at first glance. The original 19th-century North American definition, which is still used today, is saddle girth, the straps that keep a horse's saddle in place. As a verb, cinch means to pull tight, the way you'd cinch a belt. Informally, to cinch is to make absolutely certain: It'll cinch her college decision if that school offers a scholarship.
cinch, meaning of cinch in Longman Dictionary of Contemporary English
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CINCH definition in the Cambridge English Dictionary