'Nonchalant' is literally 'not warm' — from Latin 'calere' (to be hot). Cool composure, temperature-style.
Feeling or appearing casually calm and relaxed; not displaying anxiety or interest.
From French nonchalant, present participle of nonchaloir (to be indifferent, to disregard), built from non- (not) + chaloir (to matter, to be of concern). Chaloir comes from Latin calēre (to be warm, to be hot, to glow), from Proto-Indo-European *kelh₁- (to be warm). The metaphor is thermal: to care deeply is to be heated, inflamed, or fired up; to be nonchalant is to be un-warmed, cool, emotionally unheated. This heat-as-passion metaphor is deep in Indo-European languages — compare English ardent (burning