Telling someone something to make them feel confident or less worried; making something certain.
From Old French 'asseurer,' combining 'à' (to) and 'seur' (sure). The word entered Middle English meaning to make something certain or secure. It evolved to include the meaning of reassuring someone emotionally.
Assuring someone actually comes from the same root as 'sure' and 'security'—so when you assure someone, you're literally trying to make them feel secure and sure about something.
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