The story
From war + wood. Perhaps at least partly an allusion to koa (Acacia koa), a tree endemic to Hawaii with wood similar in quality to Juglans nigra, black walnut, and whose name in Hawaiian can also mean warrior, or to beefwood (Casuarina equisetifolia) which also has deep-colored, hard wood and in some Polynesian languages shares the same association between the name and words for warriors (both cognate with the Hawaiian term).
Etymology from Wiktionary, CC BY-SA 4.0.