(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.