To calm someone down or make their pain or worry go away, usually through gentle comfort or soothing actions.
From Old English 'soðian' meaning 'to prove true' or 'verify,' which evolved to mean 'to calm or pacify' because truth and certainty bring peace. The modern sense developed in Middle English as the word shifted from 'proving' to 'comforting through reassurance.'
The word originally meant 'to tell the truth to' in Old English—because apparently telling someone the truth was so comforting it became synonymous with calming them down! This shows how ancient people believed honesty and reassurance were essentially the same thing.
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