Green’s Dictionary of Slang

jock n.2

also jockster
[abbr.]

1. [early 19C+] a jockey.

2. in attrib. use of sense 1.

3. [1910s] (US) an autombile driver.

4. [1940s+] (orig. US) a disc jockey.

5. [1960s+] (US) a worker, an operator, e.g. construction jock, elevator jock [abbr. jockey n.2 (3b)].