The Collaborative International Dictionary of English v.0.48:

Stableboy \Sta"ble*boy`\ (-boi`), Stableman \Sta"ble*man\
   (-m[a^]n), n.
   A boy or man who attends in a stable; a groom; a hostler.
   [1913 Webster]

WordNet (r) 3.0 (2006):

stableboy
    n 1: someone employed in a stable to take care of the horses
         [syn: stableman, stableboy, groom, hostler,
         ostler]