From Longman Dictionary of Contemporary Englishnonchalantnon‧cha‧lant /ˈnɒnʃələnt $ ˌnɑːnʃəˈlɑːnt/ adjective DON'T CAREbehaving calmly and not seeming interested in anything or worried about anything ‘Has he got a girlfriend?’ Jill asked, trying to sound nonchalant. —nonchalance noun [uncountable] —nonchalantly adverb He smiled nonchalantly.
Examples from the Corpusnonchalant• When Peter came in, she glanced up, trying to appear nonchalant.• All her energies were devoted to appearing in the eyes of others as nonchalant.• Hoomey stood up, trying to look nonchalant.• Perkins was nonchalant about being chosen.• The idea of staying in Oregon was beginning to appeal to her, but Roy was nonchalant about it.• Well, if he does, he's pretty nonchalant about it.• a nonchalant attitude• He looked nonchalant enough as he strolled along the Bayswater Road, but inwardly Creed was a mess of nerves.• "I'm sorry I'm so late. Have you been waiting long?" he asked. She gave a nonchalant shrug.• Just around the block, swinging his arms with nonchalant vehemence.• A previously nonchalant world media took notice.Origin nonchalant (1700-1800) French Old French nonchaloir “to pay no attention to”, from Latin calere “to be warm”