An annual Old World plant (Isatis tinctoria) in the mustard family, formerly cultivated for its leaves that yield a blue dye, one of the most valuable dyestuffs. (or a good green dye when combined with a yellow dye). Said to have mild hallucinogenic properties, and usually mixed with other psychoactive substances when used as body paint (such as when going into war). Is absorbed through the skin.
Incidentally, it is also what the kid from H.R. Puff-n-stuff says instead of road.