(1):
(n.) A board which supports the back wen one is sitting;
(2):
(n.) A board serving as the back part of anything, as of a wagon.
(3):
(n.) A thin stuff used for the backs of framed pictures, mirrors, etc.
(4):
(n.) 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.
(5):
(n.) A board worn across the back to give erectness to the figure.