Extremely cold, hard, or unmerciful in temperament; completely lacking compassion or warmth.
Compound of 'flint' (hard stone) and 'hearted' (having a heart or emotional core). Metaphorically equates emotional coldness with stone hardness, common in English.
The metaphor 'hard-hearted' goes back centuries because emotions are literally linked to the heart in English—we have 'soft-hearted,' 'cold-hearted,' 'warm-hearted,' all using the heart as the site of feeling.
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