As an adjective, calm means quiet, peaceful, and not excited or upset. As a noun, it refers to a state of peace or stillness, and as a verb, it means to make someone or something less upset or noisy.
From Old French 'calme,' from Late Latin 'cauma' meaning 'heat of the day' and then 'rest during the heat,' from Greek 'kauma' meaning 'burning heat.' The idea shifted from resting during extreme heat to general peaceful stillness. It came into English in the late Middle Ages.
Originally, 'calm' was about the quiet pause forced by blazing midday heat. Sailors later used it for windless seas, and from there it spread to emotions—feelings as weather. When you 'keep calm,' you’re borrowing a word that once meant surviving the hottest, stillest part of the day.
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