Able to stay calm and make good decisions even when stressed or in a dangerous situation; not easily panicked.
Compound of 'cool' (composed, calm, from the metaphorical use of temperature for emotion) and 'headed' (possessing a type of mind or character). This phrase originated in the 17th-18th centuries as 'cool-head' before becoming 'coolheaded.'
The metaphor of 'cool' for calmness might come from the opposite—hot-headed anger literally makes your head feel warm and red with blood, so 'cool' naturally meant the opposite emotional state.
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