The Collaborative International Dictionary of English v.0.48:

Tailboard \Tail"board`\, n.
   The board at the rear end of a cart or wagon, which can be
   removed or let down, for convenience in loading or unloading.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tailboard
    n 1: a gate at the rear of a vehicle; can be lowered for loading
         [syn: tailgate, tailboard]