to pull something such as a belt tightly around something
Origin and usage
The noun cinch originally referred to a type of wide belt used to hold a saddle on a horse, used in Mexico and the southern United States. It comes from the Spanish word ‘cincha’ and both it and the related verb date from the mid 19th century.
The type of saddle girth referred to as a cinch, usually made of horsehair, was in common use in Mexico and the states of the US that adjoin it in the 19th century. It was a short step from that meaning to using the verb to refer to a belt being pulled tightly around something, the most common use today. In American English, the verb also means to make something certain to happen; the more usual verb is ‘clinch‘, which is easily confused with cinch but comes from a completely different root. The noun cinch quickly acquired figurative meanings: a cinch is something that is easy to do or something that is certain to happen. You can see examples of all these current uses below.
“This cake is delicious, and a cinch to make.”
“It’s a cinch that you’ll beat them.”
“She cinched a black-and-gray leather belt around her waist.”
“The candidate’s rousing speech cinched her nomination.”