jockstrap (n.)

also jock-strap, "supporter of the male genital organs, used in sports," 1887, with strap (n.) + jock slang for "penis" c. 1650-c. 1850, probably one of the many colloquial uses of Jock (the northern and Scottish form of Jack), which was used generically for "common man" from c. 1500. Jockey-strap in the same sense is from 1890, with also an example from 1870 but the sense is uncertain.