Etymology
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sclero- 

before vowels scler-, word-forming element meaning "hard," from Latinized form of Greek sklēros "hard, harsh," which is related to skellein "to dry up, parch" (from PIE *skle-ro-, from root *skele- "to parch, wither").

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-hearted 
figurative element in combinations, "at heart," also "having a heart" (of a specified kind), c. 1200, first attested in hard-hearted; see heart (n.). Related: -heartedly.
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karyo- 
before vowels kary-, word-forming element used since c. 1874 in biological terms referring to cell nuclei, from Greek karyon "nut, kernel," possibly from PIE root *kar- "hard," but Beekes leans toward the notion that it is a Pre-Greek word.
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