"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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