The Collaborative International Dictionary of English v.0.48:

buffalo soldier \buffalo soldier\
   A black soldier of the United States army who served in the
   American west in the late 1800's, often as an indian fighter,
   and usually as part of an all-black troop; -- the name was
   given by the indians due to their their kinky hair, and the
   name was believed also to be a compliment on their courage.
   [PJC]