The Collaborative International Dictionary of English v.0.48:

cowpoke \cowpoke\ n.
   a hired hand who tends cattle and performs other duties on
   horseback; -- an informal name for cowboy.

   Syn: cowboy, cowpuncher, puncher, cowman, cattleman, cowhand,
        cowherd.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

cowpoke
    n 1: a hired hand who tends cattle and performs other duties on
         horseback [syn: cowboy, cowpuncher, puncher,
         cowman, cattleman, cowpoke, cowhand, cowherd]