insouciance
Concise Oxford English Dictionary © 2008 Oxford University Press:
insouciant /ɪnˈsuːsɪənt/
▶adjective casually unconcerned.
– derivatives
insouciance noun,
insouciantly adverb.
insouciance noun,
insouciantly adverb.
– origin C18: from Fr., from in- ‘not’ + souciant ‘worrying’ (pres. part. of soucier).
'insouciance' also found in these Oxford entries:

