Chiefly in plural. Ashes produced by the burning of various organic substances, specifically woad plants (Isatis tinctoria), tartar left in empty wine casks, or wood. Also: a type of crude potash obtained from such ashes, especially used in dyeing cloth, wool, etc., and to make lye.
Late Middle English. From woad + ash, probably after Middle Dutch weedassche (Dutch weedas) and Middle Low German wēdassche.