If you want to understand the true meaning of fetid, leave your sweaty gym clothes in your locker for a few days. Fetid is a fancy way of saying that something smells really bad.
From the Latin word meaning "stinking," this adjective has been in use since the early 15th century, which was a particularly fetid time in history — showers, laundry detergent, and deodorant had not yet been invented. Here's an easy way to remember it: "the fe(e)t (d)id stink." It's sometimes spelled foetid.