"board made of two or more thin layers of wood bonded together and arranged so that the grain of one runs at right angles to that of the next," 1907, from ply (n.) + wood (n.).

"a layer, a fold," 1530s, from French pli "a fold" (13c.), alteration of Old French ploi "fold, pleat, layer" (12c.), verbal noun from ployer (later pleier) "to bend, to fold," from Latin plicare "to fold, lay" (from PIE root *plek- "to plait"). Often used to indicate the number of thicknesses of which anything is made; this also is the ply in plywood.