Source : Webster's Revised Unabridged Dictionary (1913)

Jockey \Jock"ey\, n.; pl. {Jockeys}. [Dim. of Jack, Scot. Jock;
   orig., a boy who rides horses. See 2d {Jack}.]
   1. A professional rider of horses in races. --Addison.

   2. A dealer in horses; a horse trader. --Macaulay.

   3. A cheat; one given to sharp practice in trade.