The Collaborative International Dictionary of English v.0.48:

Highwayman \High"way`man\, n.; pl. Highwaymen.
   One who robs on the public road; a highway robber.
   [1913 Webster]

WordNet (r) 3.0 (2006):

highwayman
    n 1: a holdup man who stops a vehicle and steals from it [syn:
         highjacker, highwayman, hijacker, road agent]