nonchalant
[adjective]
Synonyms: casual, blasé, calm, careless, indifferent, insouciant, laid-back (informal), offhand, unconcerned, unperturbed

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Contrasted words: anxious, careful, concerned, solicitous, worried
Related Words: cheerful, glad, lighthearted, easy, effortless, light, smooth

[TahlilGaran] English Synonym Dictionary ▲

non‧cha‧lant /ˈnɒnʃələnt $ ˌnɑːnʃəˈlɑːnt/ adjective
[Date: 1700-1800; Language: French; Origin: Old French nonchaloir 'to pay no attention to', from Latin calere 'to be warm']
behaving 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.

[TahlilGaran] Dictionary of Contemporary English ▲