To become or make firm, solid, or resistant to pressure. To become less sympathetic, flexible, or emotionally responsive.
From Old English 'heardian', derived from 'heard' (hard), from Proto-Germanic 'hardhuz'. The metaphorical sense of becoming emotionally tough developed in Middle English, extending the physical concept of solidification to psychological states. The word reflects the human tendency to conceptualize emotional states through physical metaphors.
Metallurgy has given us multiple ways to harden materials - from tempering steel through rapid cooling to case hardening by adding carbon to the surface. Psychologically, we use the same word for emotional resilience, reflecting an ancient understanding that minds and metals both can be strengthened through controlled stress and recovery.
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