The Collaborative International Dictionary of English v.0.48:

Jockey \Jock"ey\, n.; pl. Jockeys. [Dim. of Jack, Scot. Jock;
   orig., a boy who rides horses. See 2d Jack.]
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   1. A professional rider of horses in races. --Addison.
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   2. A dealer in horses; a horse trader. --Macaulay.
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   3. A cheat; one given to sharp practice in trade.
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