plank: [13] The etymological idea underlying plank may be ‘flatness’. It comes via planke, a northern dialect version of Old French planche (source of English planchette [19]), from late Latin planca ‘slab’, a derivative of the adjective plancus ‘flat’. This may have come from the same source as Greek pláx ‘flat surface’, ancestor of English placenta. => planchette[plank etymology, plank origin, 英语词源]