1952, short for jockstrap "supporter of the male genital organs, used in sports" (1897), from jock slang for "penis" c.1650-c.1850, probably from Jock, the nickname for John, which was used generically for "common man" from 1508. Meaning "an athletic man" is from 1963, Amer.Eng. slang.
The penis; the crotch: I'll be beating the bushes with snow to my jock(1790+)
An athletic supporter; jockstrap: I asked him if he wanted some sweat clothes, or a jock(1952+)
(also jocko) An athlete •Now used of both men and women, despite the phallic derivation: The players themselves are a curious blend of woman and jock/ the lucrative job proper to an all-Ivy jock(1963+)
[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]
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
jock in Technology
1. A programmer who is characterised by large and somewhat brute-force programs. 2. When modified by another noun, describes a specialist in some particular computing area. The compounds "compiler jock" and "systems jock" seem to be the best-established examples. [Jargon File] (1995-01-19)