Etymology
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harden (n.)
c. 1200, transitive, "make (something) hard," from hard (adj.) + -en (1). Intransitive meaning "to become hard" is late 14c. The earlier verb was simply hard, from Old English heardian. Related: Hardened; hardening.
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hardened (adj.)
past-participle adjective from harden (v.). Figurative sense of "unfeeling" is from late 14c.
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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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callus (n.)
"hardened skin," 1560s, from Latin callus, variant of callum "hard skin," related to callere "be hard," from Proto-Italic *kaln/so- "hard;" PIE source uncertain. Likely cognates are Old Irish calath, calad, Welsh caled "hard;" Old Church Slavonic kaliti "to cool, harden," Russian kalit "to heat, roast," Serbo-Croatian kaliti "to temper, case-harden."
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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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endure (v.)
late 14c., "to undergo or suffer" (especially without breaking); also "to continue in existence," from Old French endurer (12c.) "make hard, harden; bear, tolerate; keep up, maintain," from Latin indurare "make hard," in Late Latin "harden (the heart) against," 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."

Replaced the important Old English verb dreogan (past tense dreag, past participle drogen), which survives in dialectal dree. Related: Endured; endures.
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obdurate (adj.)

mid-15c., "stubborn, inexorable, unyielding; hardened," especially against moral influences; "stubbornly wicked," from Latin obduratus "hardened," past participle of obdurare "harden, render hard; be hard or hardened; hold out, persist, endure," in Church Latin "to harden the heart against God," from ob "against" (see ob-) + durare "harden, render hard," from durus "hard," from PIE *dru-ro-, suffixed variant form of root *deru- "be firm, solid, steadfast." Variant opturate is from early 15c. in medicine in a literal sense of "stopped, obstructed."  Related: Obdurately; obdurateness.

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obduration (n.)

c. 1400, obduracioun, "hard-heartedness; defiant impenitence," from Late Latin obdurationem (nominative obduratio) "a hardening," noun of state from past-participle stem of Latin obdurare "harden, render hard; be hard or hardened" (see obdurate).

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bronze (v.)
1640s, "give the color or appearance of bronze to," from French bronzer (16c.) or else from bronze (n.). Figuratively, of feelings, hearts, etc., "to harden like bronze," 1726. Meaning "to make to be brown or bronze in color" (by exposure to the sun, etc.) is from 1792. Related: Bronzed; bronzing.
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