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.
[the basic etymon is jock, ''penis,'' fr jack, probably the diminutive of John, which fr the 14th century has been applied to males, malelike things, and male organs; the sense ''athlete'' is fr jockstrap]