The Collaborative International Dictionary of English v.0.48:

Buckboard \Buck"board`\, n.
   A four-wheeled vehicle, having a long elastic board or frame
   resting on the bolsters or axletrees, and a seat or seats
   placed transversely upon it; -- called also buck wagon.
   [1913 Webster]

WordNet (r) 3.0 (2006):

buckboard
    n 1: an open horse-drawn carriage with four wheels; has a seat
         attached to a flexible board between the two axles