In a calm, unexcited, or unemotional manner; with a cool and detached attitude.
From cool + -ly (adverb suffix). 'Cool' comes from Old English 'col' meaning low in temperature, which extended metaphorically to emotional temperature.
The phrase 'playing it cool' actually works—research shows that acting calm during stressful moments actually makes you calmer because your body follows your behavior through feedback loops.
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