The Collaborative International Dictionary of English v.0.48:

Roadster \Road"ster\, n.
   1. (Naut.) A clumsy vessel that works its way from one
      anchorage to another by means of the tides. --Ham. Nav.
      Encyc.
      [1913 Webster]

   2. A horse that is accustomed to traveling on the high road,
      or is suitable for use on ordinary roads.
      [1913 Webster]

            A sound, swift, well-fed hunter and roadster.
                                                  --Thackeray.
      [1913 Webster]

   3. A bicycle or tricycle adapted for common roads rather than
      for the racing track.
      [1913 Webster]

   4. One who drives much; a coach driver. [Eng.]
      [1913 Webster]

   5. A hunter who keeps to the roads instead of following the
      hounds across country. [Eng. Slang.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

roadster
    n 1: an open automobile having a front seat and a rumble seat
         [syn: roadster, runabout, two-seater]
    2: a small lightweight carriage; drawn by a single horse [syn:
       buggy, roadster]