nonchalant

Synonyms: airy aloof apathetic blase calm careless casual cold collected composed cool detached disimpassioned disinterested dispassionate easy effortless happy impassive imperturbable incurious indifferent insouciant lackadaisical light listless loose lukewarm mellow neglectful negligent neutral offhand placid serene smooth trifling uncaring unconcerned unemotional unexcited unfeeling unflappable unimpressible unperturbed unruffled untroubled

Antonyms: intense jumpy nervous unnerved