Insouciant (adjective)

1. Carefree and not worried; nonchalant.
2. Appearing indifferent or unconcerned about something.

Origin:

This word comes from the french word insouciant, which means "carefree, nonchalant", formed by in- meaning "not" and souci meaning "care".

Examples:

1. He remained insouciant despite the crisis.
2. Her insouciant attitude towards the exam was not appreciated by the teacher.
3. The insouciant smile on his face gave no indication of his true feelings.
4. The insouciant response to the question raised suspicions.
5. The insouciant manner in which he handled the situation impressed everyone.