When the word cinch was borrowed into English in 1859, it described a type of Mexican saddle girth. The girths were typically considered to be pretty tight, so by the 1880s, cinch was used to describe anything that was firm or secure. That figuratively developed further into the "easy thing" definition that we know from the word's usage today, since it's easy to do something when it's secure. Cinch comes from Spanish cincha, which traces to Latin cingulum, meaning "girdle" or "swordbelt". That traces to the verb cingere, "to encircle" and the Proto-Indo-European root kenk (with the same translation). The verb form of cinch (meaning "to make certain") is from the 1890s, the name of the card game might actually be from Spanish cinco, and literary usage of the term peaked in 1919.
Hello! I'm Adam Aleksic, a sophomore studying linguistics and government at Harvard University, where I founded the Harvard Undergraduate Linguistics Society. I also have disturbing interests in politics, vexillology, geography, board games, conlanging, and law.