From Longman Dictionary of Contemporary Englishinsouciancein‧sou‧ci‧ance /ɪnˈsuːsiəns/ noun [uncountable] formal HAPPYa cheerful feeling of not caring or worrying about anything SYN nonchalance an air of insouciance —insouciant adjective —insouciantly adverb
Examples from the Corpus
insouciance• For all her apparent insouciance, she was desperately unhappy.Origin insouciance (1800-1900) French soucier “to trouble” 