Etymology
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indurate (v.)
1590s (transitive) "make hard;" 1620s (intransitive) "grow harder," 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." Related: Indurated.
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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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*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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