In a way that is insensitive, cruel, or showing a lack of concern for others' feelings or suffering.
From the adjective 'callous' (emotionally hardened) with the adverbial suffix '-ly.' The word evolved its metaphorical meaning from physical hardness to moral hardness.
The etymology shows how medieval people understood emotion: they imagined the heart and conscience as soft, tender things that could literally harden like skin if repeatedly wounded or ignored.
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