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]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "washboard":
   Tarmac, Tarvia, asphalt, bitumen, bituminous macadam, blacktop,
   brick, broken ground, broken water, cement, chop, cobble,
   cobblestone, concrete, corduroy, corduroy road, corrugation, curb,
   curbing, curbstone, edgestone, flag, flagging, flagstone,
   goose bumps, goose pimples, gooseflesh, gravel, horripilation,
   kerb, kerbstone, lop, macadam, pavement, pavestone, paving,
   paving stone, ripple, road metal, rough, sandpaper, stone,
   tarmacadam, tile