As an adjective, callous describes someone who is emotionally hard and does not care about other people's feelings. As a verb (rare), it means to make something hard or insensitive, like skin becoming thick from repeated rubbing.
From Latin 'callosus' meaning 'thick-skinned, hardened,' from 'callum' meaning 'hard skin.' English first used it for physical hardening of skin and later for emotional hardness. The figurative sense of lacking sympathy became common in modern English.
The same root that gives us 'callus' on your hand also gives us 'callous' for a cold-hearted person. It’s a vivid metaphor: just as skin can lose feeling when it thickens, a person can lose sensitivity when they emotionally harden. Language quietly links physical and emotional numbness.
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