What does "woad" mean?

Woad: 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 It is now superseded by indigo, but is somewhat used with indigo as a ferment in dyeing.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.