Etymology
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*deru- 

also *dreu-, Proto-Indo-European root meaning "be firm, solid, steadfast," with specialized senses "wood," "tree" and derivatives referring to objects made of wood.

It forms all or part of: betroth; Dante; dendrite; dendro-; dendrochronology; dour; Druid; drupe; dryad; dura mater; durable; durance; duration; duress; during; durum; endure; hamadryad; indurate; obdurate; perdurable; philodendron; rhododendron; shelter; tar (n.1) "viscous liquid;" tray; tree; trig (adj.) "smart, trim;" trim; troth; trough; trow; truce; true; trust; truth; tryst.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dru "tree, wood," daru "wood, log, timber;" Greek drys "oak," drymos "copse, thicket," doru "beam, shaft of a spear;" Old Church Slavonic drievo "tree, wood," Serbian drvo "tree," drva "wood," Russian drevo "tree, wood," Czech drva, Polish drwa "wood;" Lithuanian drūtas "firm," derva "pine, wood;" Welsh drud, Old Irish dron "strong," Welsh derw "true," Old Irish derb "sure," Old Irish daur, Welsh derwen "oak;" Albanian drusk "oak;" Old English treo, treow "tree," triewe "faithful, trustworthy, honest."

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Derry 

frequent element in Irish place-names, from Irish doire "an oak wood," from Old Irish daur "oak," from PIE root *deru- "tree," especially oak.

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dour (adj.)

mid-14c., "severe" (of grief); late 14c., of men, "bold, stern, fierce," a word from Scottish and northern England dialect, probably directly from Latin durus "hard," from PIE *dru-ro-, suffixed variant form of root *deru- "be firm, solid, steadfast." Sense of "gloomy, sullen" is late 15c. Related: Dourness.

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dendro- 

word-forming element meaning "tree," from Greek dendron "tree," sometimes especially "fruit tree" (as opposed to hylē "timber"), from PIE *der-drew-, from root *deru- "to be firm, solid, steadfast," also forming words for "wood, tree."

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dendrite (n.)

"natural marking found on some stones in the form of branching shrubs, trees, or mosses," 1745, from Greek dendrites "of or pertaining to a tree," from dendron "tree," from PIE *der-drew-, from root *deru- "to be firm, solid, steadfast," also forming words for "wood, tree."

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trig (adj.)
"smart, trim," c. 1200, from Old Norse tryggr "firm, trusty, true," from Proto-Germanic *treuwaz "having or characterized by good faith," from PIE *drew-o-, a suffixed form of the root *deru- "be firm, solid, steadfast." A Scottish and northern word only until 19c. Related: Trigness.
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indurate (adj.)
"hardened, made hard," early 15c., from Latin induratus, past participle of indurare "to make hard, harden," from in- (from PIE root *en "in") + durare "to harden," from durus "hard," from PIE *dru-ro-, suffixed variant form of root *deru- "be firm, solid, steadfast."
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tray (n.)
Old English treg, trig "flat wooden board with a low rim," from Proto-Germanic *traujam (source also of Old Swedish tro, a corn measure), from PIE *drou-, variant of root *deru- "be firm, solid, steadfast," with specialized senses "wood, tree" and derivatives referring to objects made of wood. The primary sense may have been "wooden vessel."
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dryad (n.)

nymph of the woods, 1550s (plural Driades is attested from late 14c.), from Latin dryas, from Greek dryas (plural dryades) "wood nymph," from drus (genitive dryos) "oak," from PIE root *deru- "be firm, solid, steadfast," with specialized senses "wood, tree." Related: Dryadic.

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Dante 

masc. proper name, most modern uses outside Italy ultimately are in reference to Dante Alighieri (c. 1265-1321), the great poet; the name is a shortening of Latin Durante, from durare "to harden, endure," from durus "hard," from PIE *dru-ro-, suffixed variant form of root *deru- "be firm, solid, steadfast." Related: Dantean, Dantescan, Dantesque (the last from French).

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