'cool-headedness' originates from Modern English, formed by the adjective 'cool-headed' plus the nominalizing suffix '-ness'. 'cool-headed' itself is a compound of 'cool' and 'headed'.

'cool' comes from Old English 'cōl' (from Proto-Germanic *kōlaz) meaning 'not warm'; 'head' derives from Old English 'hēafod' (from Proto-Germanic *haubudam) meaning 'head'. The adjective 'cool-headed' (calm in temperament) developed by combining those elements, and adding the Old English-derived suffix '-ness' (from Old English '-nes(s)e') produced the noun 'cool-headedness' in later Modern English.

The original elements referred to physical 'cool' and the anatomical 'head', but the compound came to mean 'having a calm, composed mind'; over time the noun form 'cool-headedness' came to denote the quality of being calm under pressure.