The Collaborative International Dictionary of English v.0.48:

Washboard \Wash"board`\, n.
   1. A fluted, or ribbed, board on which clothes are rubbed in
      washing them.
      [1913 Webster]

   2. A board running round, and serving as a facing for, the
      walls of a room, next to the floor; a mopboard.
      [1913 Webster]

   3. (Naut.) A broad, thin plank, fixed along the gunwale of
      boat to keep the sea from breaking inboard; also, a plank
      on the sill of a lower deck port, for the same purpose; --
      called also wasteboard. --Mar. Dict.
      [1913 Webster]

WordNet (r) 3.0 (2006):

washboard
    n 1: device consisting of a corrugated surface to scrub clothes
         on
    2: protective covering consisting of a broad plank along a
       gunwale to keep water from splashing over the side [syn:
       washboard, splashboard]