A board which supports the back when one is sitting; [1913 Webster]
A board serving as the back part of anything, as of a wagon. [1913 Webster]
A thin stuff used for the backs of framed pictures, mirrors, etc. [1913 Webster]
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]
A board worn across the back to give erectness to the figure. Thackeray. [1913 Webster]