'A board is thinner than a plank, and generally less than 2.5 inches thick. The transferred meaning "food" (late 14c.) is an extension of the late Old English sense of "table" (cf. boarder, boarding); hence, also, above board "honest, open" (1610s). A further extension is to "table where council is held" (1570s), then transferred to "leadership council, council (that meets at a table)," 1610s.'
Takovýhle vysvětleníčko existuje skoro pro každé slovo a je záhodno je všechna znát. Aby bylo jasné, že někdo kdo je "thick as a plank" je příliš tlustý na to, být ředitelské prkno