thin board; sheet; thin plate
Refers to a thin, flat piece of material such as wood, metal, or plastic. The phrase is a simple noun phrase composed of the adjective 薄い (thin) and the noun 板 (board/plate).
This thin board is plywood.
I cut a thin board and made a shelf.
A straightforward compound of 薄い (thin) and 板 (board/plate). No special etymology.