Insouciant
Definition: unconcerned; blithe, to the point of carelessness
It’s a negation of the French word “soucier,” which means “to bother or trouble.”
Why I Looked It Up
From Messy: The Power of Disorder to Transform Our Lives:
Hitler’s insouciant remark that “we’ll have to do without science for a few years” was as self-destructive in his day as it would be in ours.
Update
Added on
From Saturday Night: A Backstage History of Saturday Night Live, when discussing Chevy Chase being nonchalant about his growing fame:
Opportunity for such insouciance soon faded as the evidence of his celebrity became too obvious to ignore.