Source : Webster's Revised Unabridged Dictionary (1913)

Cowboy \Cow"boy`\ (-boi`), n.
   1. A cattle herder; a drover; specifically, one of an
      adventurous class of herders and drovers on the plains of
      the Western and Southwestern United States.

   2. One of the marauders who, in the Revolutionary War
      infested the neutral ground between the American and
      British lines, and committed depredations on the
      Americans.

Source : WordNet®

cowboy
     n 1: a hired hand who tends cattle and performs other duties on
          horseback [syn: {cowpuncher}, {puncher}, {cowman}, {cattleman},
           {cowpoke}, {cowhand}, {cowherd}]
     2: a performer who gives exhibitions of riding and roping and
        bulldogging [syn: {rodeo rider}]
     3: someone who is reckless or irresponsible (especially in
        driving vehicles)

Source : Free On-Line Dictionary of Computing

cowboy
     
        [Sun, from William Gibson's {cyberpunk} SF]  Synonym
        for {hacker}.  It is reported that at Sun this word is often
        said with reverence.
     
        [{Jargon File}]