nonchalant

adjective

1. coolly unconcerned, indifferent, or unexcited; casual: His nonchalant manner infuriated me.

adjective

1. casually unconcerned or indifferent; uninvolved

adj.1734, from French nonchalant, present participle of nonchaloir “be indifferent to, have no concern for” (13c.), from non- “not” (see non-) + chaloir “have concern for,” ultimately from Latin calere “be hot” (see calorie). French chaland “customer, client” is of the same origin. Related: Nonchalantly.