The word is Insouciant which means: in a lighthearted, unconcerned manner: nonchalance.
Here is the entry from Merriam Webster online...
- Main Entry: in·sou·ci·ance , noun, : lighthearted unconcern : nonchalance
- Pronunciation: \in-ˈsü-sē-ən(t)s, aⁿ-süs-ˈyäⁿs\
- Etymology: French, from in- + soucier to trouble, disturb, from Old French, from Latin sollicitare — more at solicit
- Date: 1799
— in·sou·ci·ant \in-ˈsü-sē-ənt, aⁿ-süs-yäⁿ\ adjective
— in·sou·ci·ant·ly \in-ˈsü-sē-ənt-lē\ adverb