Old English treo, treow "tree" (also "wood"), from Proto-Germanic *trewan (cf. Old Frisian tre, Old Saxon trio, Old Norse tre, Gothic triu), from PIE *deru- "oak" (cf. Sanskrit dru "tree, wood," daru "wood, log;" Greek drys "oak," doru "spear;" Old Church Slavonic drievo "tree, wood;" Serbian drvo "tree," drva "wood;" Russian drevo "tree, wood;" Czech drva; Polish drwa "wood;" Lithuanian derva "pine wood;" Old Irish daur, Welsh derwen "oak," Albanian drusk "oak").
Importance of the oak in mythology is reflected in the recurring use of words for "oak" to mean "tree." In Old English and Middle English, also "thing made of wood," especially the cross of the Crucifixion and a gallows (cf. Tyburn tree, gallows mentioned 12c. at Tyburn, at junction of Oxford Street and Edgware Road, place of public execution for Middlesex until 1783). Sense in family tree first attested 1706; verb meaning "to chase up a tree" is from 1700. Tree-hugger, contemptuous for "environmentalist" is attested by 1989.
