The Collaborative International Dictionary of English v.0.48:

Clapboard \Clap"board\, v. t.
   To cover with clapboards; as, to clapboard the sides of a
   house. [U. S.] --Bartlett.
   [1913 Webster] Clapbread

The Collaborative International Dictionary of English v.0.48:

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.]
      [1913 Webster]

   2. A stave for a cask. [Eng.] --Halliwell.
      [1913 Webster]

WordNet (r) 3.0 (2006):

clapboard
    n 1: a long thin board with one edge thicker than the other;
         used as siding by lapping one board over the board below
         [syn: clapboard, weatherboard, weatherboarding]
    v 1: cover with clapboards