Old English wudu, earlier widu "tree, trees collectively, the substance of which trees are made," from Proto-Germanic *widuz (cf. Old Norse viðr, Danish and Swedish ved "tree, wood," Old High German witu "wood"), perhaps from PIE *widhu- "tree, wood" (cf. Welsh gwydd "trees," Gaelic fiodh- "wood, timber," Old Irish fid "tree, wood"). Woodsy is from 1860. Out of the woods "safe" is from 1792.
"violently insane" (now obsolete), from Old English wod "mad, frenzied," from Proto-Germanic *woth- (cf. Gothic woþs "possessed, mad," Old High German wuot "mad, madness," German wut "rage, fury"), from PIE *wet- "to blow, inspire, spiritually arouse;" source of Latin vates "seer, poet," Old Irish faith "poet;" "with a common element of mental excitement" [Buck]. Cf. Old English woþ "sound, melody, song," and Old Norse oðr "poetry," and the god-name Odin.
The thick xylem of trees and shrubs, resulting from secondary growth by the vascular cambium, which produces new layers of living xylem. The accumulated living xylem is the sapwood. The older, dead xylem in the interior of the tree forms the heartwood. Often each cycle of growth of new wood is evident as a growth ring. The main components of wood are cellulose and lignin.
Some people say, “Knock on wood,” and then knock on something made of wood for good luck, when they have made a remark that has been true up to that point and they want it to continue to be true: “I've never had an accident yet, knock on wood.”
Note: The expression alludes to an ancient superstition that touching wood would ward off evil spirits.