backboard

Back"board`, n. Etym: [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.

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