Source : Webster's Revised Unabridged Dictionary (1913)

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}.

Source : WordNet®

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