Backboard

Part of speech: noun (n.)

1. A board which supports the back wen one is sitting;

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.