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unconcerned, undisturbed; carefree and nonchalant
Latin in- + soucier 'to disturb'
1829, from French insouciant "careless, thoughtless, heedless," from in- "not" (see in- (1)) + souciant "caring," present participle of soucier "to care," from Latin sollicitare "to agitate" (see solicit). Related: Insouciantly.