Synonyms for nonchalant

unconcerned, casual, indifferent, insouciant, untroubled, cool, imperturbable, easygoing, lackadaisical, apathetic, lukewarm, unruffled, unexcited, composed, collected, aloof, detached, offhand, unexcitable, unflappable, calm, serene, placid, incurious, disinterested, easy, effortless, relaxed, light, smooth, neutral, unimpressible, blasé, devil-may-care, heedless, regardless, careless, laid-back*; see also calm 1, careless 1, unconcerned.

Antonyms warm*, ardent, enthusiastic. See syn. study at cool.cool.