Cruel, unkind, and lacking sympathy or compassion for others' suffering.
Compound of 'hard' and 'hearted', using the heart as the metaphorical seat of emotions and compassion. Since at least medieval times, a 'hard heart' has represented emotional coldness and cruelty.
Interestingly, neuroscience suggests people who display extreme hardheartedness often have reduced activity in brain regions associated with empathy—their cruelty may be partially neurological rather than purely chosen.
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