The Collaborative International Dictionary of English v.0.48:

Busby \Bus"by\ (b[u^]z"b[y^]), n.; pl. Busbies (b[i^]z).
   (Mil.)
   A military headdress or cap, used in the British army. It is
   of fur, with a bag, of the same color as the facings of the
   regiment, hanging from the top over the right shoulder.
   [1913 Webster]

WordNet (r) 3.0 (2006):

busby
    n 1: tall hat; worn by some British soldiers on ceremonial
         occasions [syn: bearskin, busby, shako]