insouciance [n-soo-see-uh ns; French an-soo-syahns]
1 Casual lack of concern; lighthearted unconcern, nonchalance
2 Lack of concern shown by someone about something which they might be expected to take more seriously.
On this tune the tenor saxophonist Chad Lefkowitz-Brown improvises with a combination of breezy insouciance and sharp rhythmic assurance. (Jon Pareles, The Playlist: The 1975 Talk About Its Generation, and 9 More New Songs)
The ensemble exuded carefree insouciance and managed to look polished without feeling overdone, which is never a good look, no matter what the season. (Maria Ward, How Sienna Miller Moves the Bedroom Slipper to the Street)
She had a gaiety and insouciance , and a natural childlike merriment that all her terrible disasters could not overcloud. (Louise Mack, A Woman's Experience in the Great War)
c 1820, from French insouciance 'heedless indifference or unconcern,' from insouciant 'carelessness, thoughtlessness, heedlessness,' from in- 'not' (see in- (1)) + souciant 'caring,' present participle of soucier 'to care,' from Latin sollicitare 'to agitate'. (Online Etymology Dictionary)
English speakers learned 'insouciance' from the French in the 1700s (and the adjective 'insouciant' has been part of our language since the 1800s). The French word comes from a combination of the negative prefix in- and soucier, meaning 'to trouble or disturb.' 'Soucier' in turn traces to 'sollicitus,' the Latin word for 'anxious.' If it seems to you that 'sollicitus' looks a lot like some other English words you've seen, you're right. That root also gave us 'solicit' (which now means 'to entreat' but which was once used to mean 'to fill with concern or anxiety'), 'solicitude' (meaning 'uneasiness of mind'), and 'solicitous' ('showing or expressing concern'). (Dictionary.com)