\Back"board`\, n. [2nd back, n. + board.]
1. A board which supports the back wen one is sitting;

Note: specifically, the board athwart the after part of a
      boat.

2. A board serving as the back part of anything, as of a
   wagon.

3. A thin stuff used for the backs of framed pictures,
   mirrors, etc.

4. A board attached to the rim of a water wheel to prevent
   the water from running off the floats or paddies into the
   interior of the wheel. --W. Nicholson.

5. A board worn across the back to give erectness to the
   figure. --Thackeray.