Why is the ninth month called September?
1520s, "boy, fellow," originally a Scottish proper name, variant of Jack. The meaning "person who rides horses in races" first attested 1660s.
1708, "trick, outwit, gain advantage," from jockey (n.) perhaps from its former additional sense of "horse trader" (1680s). Meaning "to ride a horse in a race" is from 1767. Related: Jockeyed; jockeying.