A hardened, thickened area of skin or tissue that develops in response to repeated pressure, friction, or trauma; a callus.
From Latin 'callosus' (thick, hard) through the Romance languages. The suffix '-osity' creates an abstract noun meaning the quality or condition of being callous.
The etymology reveals how Latin 'callosus' traveled through different meaning: first it meant hard skin, then it came to mean hardened in character—emotional callousness literally borrows the metaphor of toughened skin.
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