Showing no sympathy, pity, or compassion for others; being cruel or unkind.
From Old English combining 'heart' (the organ, but also emotions and compassion) and '-less' (without). This reflects ancient beliefs that the heart was the center of feelings.
We still say heartless because the heart has been the symbol of emotions for thousands of years—ancient doctors thought love and compassion literally came from the heart, so losing heart means losing compassion.
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