backboard

n. [ 2d back, n. + board. ] [ 1913 Webster ]

1. A board which supports the back when one is sitting; specifically, the board athwart the after part of a boat. [ 1913 Webster ]

2. A board serving as the back part of anything, as of a wagon. [ 1913 Webster ]

3. A thin stuff used for the backs of framed pictures, mirrors, etc. [ 1913 Webster ]

4. A board attached to the rim of a water wheel to prevent the water from running off the floats or paddles into the interior of the wheel. W. Nicholson. [ 1913 Webster ]

5. A board worn across the back to give erectness to the figure. Thackeray. [ 1913 Webster ]