Source : Webster's Revised Unabridged Dictionary (1913)

Clapboard \Clap"board\, n.
   1. A narrow board, thicker at one edge than at the other; --
      used for weatherboarding the outside of houses. [U. S.]

   2. A stave for a cask. [Eng.] --Halliwell.

Clapboard \Clap"board\, v. t.
   To cover with clapboards; as, to clapboard the sides of a
   house. [U. S.] --Bartlett.

Source : WordNet®

clapboard
     n : a long thin board with one edge thicker than the other; used
         as siding by lapping one board over the board below [syn:
          {weatherboard}, {weatherboarding}]
     v : cover with clapboards