woad /ˈwod/
  大青;菘藍;藍色染料
  Woad n.  [Written also wad, and wade.]
  1. Bot. An herbaceous cruciferous plant (Isatis tinctoria) of the family Cruciferae (syn. Brassicaceae). It was formerly cultivated for the blue coloring matter derived from its leaves.  See isatin.
  2. A blue dyestuff, or coloring matter, consisting of the powdered and fermented leaves of the Isatis tinctoria. It is now superseded by indigo, but is somewhat used with indigo as a ferment in dyeing.
     Their bodies . . . painted with woad in sundry figures.   --Milton.
  Wild woad Bot., the weld (Reseda luteola).  See Weld.
  Woad mill, a mill grinding and preparing woad.
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  woad
       n 1: a blue dyestuff obtained from the woad plant
       2: any of several herbs of the genus Isatis