1) A diminutive of John via Jock and recorded in Scotland from <i> a </i>.1529, when the meaning was pejorative. It came to be used as a name for a horse.
1693 lent him a bay horse to ride on called Jockey , Calverley and was already the word for a horse-dealer in the north of England by 1638, and a horse-rider by 1643. An extract from the Leeds Mercury is evidence of how it was used: 1726 James Browne alias Patrick Trueman was apprehended near Wakefield ... on suspition of Horse-Stealing ... But the Prisoner to prevent Jack Ketch in the Execution of his office hang’d himself ... Tis supposed this Suicide had been a considerable Dealer in Horses ... The Jockey when taken had a Couple of stout Mares . The term ‘horse jockey’ was in use by then: 1731 William Mortimer of Ovenden, horsjockey
1744 James Blakeley, Horse Jockky, Batley.