From The Collaborative International Dictionary of English v.0.48:
Woad \Woad\, n. [OE. wod, AS. w[=a]d; akin to D. weede, G. waid,
OHG. weit, Dan. vaid, veid, Sw. veide, L. vitrum.] [Written
also wad, and wade.]
[1913 Webster]
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.
[1913 Webster]
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It is now superseded by indigo, but is somewhat used with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.
[1913 Webster]
Their bodies . . . painted with woad in sundry
figures. --Milton.
[1913 Webster]
Wild woad (Bot.), the weld (Reseda luteola). See Weld.
Woad mill, a mill grinding and preparing woad.
[1913 Webster]