Insouciant

ɪnˈsuːsiənt

adjective

showing a casual lack of concern; indifferent

The word 'insouciant' comes from French, where 'in' means 'not' and 'souci' means 'care'. Therefore, it literally means 'not concerned'. It is often used to describe someone who appears unconcerned or carefree.

Before Ralph lost sight of her, she had achieved the limping yet insouciant trot which was her trademark.

Stephen King

Insomnia

‘Of course,’ Colonel Korn answered pleasantly, after he had chased the mighty guard of massive M.P.s out with an insouciant flick of his hand and a slightly contemptuous nod—most relaxed, as always, when he could be most cynical.

Heller, Joseph

Catch-22