1952, short for jockstrap "supporter of the male genital organs," which also meant, in slang, "athletic male." Jock with the meaning "an athletic man" is from 1963, American English slang. A jockette (1948) originally was a female disk jockey, then a female jockey (1969), then an athletic female (1979).
c. 1500, variant of the masc. proper name Jack, the by-form of John. In Scotland and northern England it is the usual form. Since 1520s, like Jack, it has been used generically, as a common appellative of lads and servants, as the name of a typical man of the common folk, of a Scottish or North Country seaman, etc.