Having hardened, thickened skin from repeated friction or pressure; emotionally hardened or insensitive.
From Late Latin 'callosus' (thick, hard) through Old French. The word took on the figurative meaning of emotional hardness because calluses were seen as a form of hardening.
The word's double meaning—physical hardness and emotional hardness—reflects a deep truth about humans: we literally build walls, both of skin and of emotion, in response to repeated injury or pressure.
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