doru-mn̥-
“beam, timber, spear-shaft”tree, timber, dendrite, durable
Extended form of *deru- giving English tree, timber, Greek dendron > dendrite.
Discussion
The PIE form *doru-mn̥- (beam, timber, spear-shaft) derives from the root *dóru (tree, wood, spear), one of the oldest and most culturally significant nouns in the reconstructed vocabulary. The root *dóru belongs to the archaic u-stem declension class, with an oblique stem *drew-/*dru- that produced a separate but related family of derivatives.
The primary reflex in English is tree itself, from Old English trēow (tree, wood, timber), from PGmc *trewą, from PIE *drew-o- (the o-grade of the oblique stem). The word's survival in its basic concrete meaning — unchanged in referent for five thousand years — makes it one of the most conservatively inherited nouns in English. German Treu (faithful, loyal — originally "firm as wood, solid as a tree") shows a striking semantic extension: trustworthiness conceived as the quality of timber.
Greek dóry (δόρυ, "spear, spear-shaft, beam of wood") preserves the PIE form with the specific martial sense: the tree as weapon, the wood as shaft. Homeric Greek uses dóry both for the spear itself and, by metonymy, for battle — a synecdoche in which the weapon stands for the act of fighting. The derivative drŷs (δρῦς, "oak, tree") gave English druid (through Celtic *dru-wid-, "oak-knower" or "tree-seer" — the druids as priests of the sacred oak).
Latin dūrus (hard, lasting, enduring) derives from the same root through the concept of wood's hardness: what is wooden is hard, what is hard is durable. From dūrus English receives: durable, duration, endure, during, duress (hardness applied to a person), and obdurate (hardened against, stubborn). The semantic chain wood → hard → lasting → enduring encodes a material metaphor that has outlived its origin: we speak of an enduring friendship or a durable peace without thinking of the timber that first meant "hard."
Sanskrit dā́ru (wood, timber) and the Avestan cognate confirm the Indo-Iranian reflexes. Hittite taru (tree, wood) provides Anatolian attestation with the expected preservation of the initial dental.
The word timber itself (OE timber, from PGmc *timrą) may be from a related root or an extension of *dóru, though the exact connection is debated. If related, the English words tree, timber, durable, druid, and true (OE trēowe, from the "faithful/solid" sense) all descend from a single PIE noun for the most essential material of the preindustrial world.