The Collaborative International Dictionary of English v.0.48:

Hobo \Ho"bo\, n.; pl. Hobos or Hoboes. [Of uncertain
   origin.]
   A professional tramp; one who spends his life traveling from
   place to place, esp. by stealing rides on trains, and begging
   for a living. [U. S.] -- Ho"bo*ism, n.

   Syn: tramp; bum; vagrant; knight of the road.
        [Webster 1913 Suppl.]