He kept dropping an Indian club and picking it up with forced and scowling insouciance.
-- John Banville, Eclipse, 2007
Gleason gave me a warning or two about the possible dangers into which my insouciance might yet lead me.
-- Gore Vidal, Death in the Fifth Position, 1952
The Latin root sollicitaire coupled with the negating prefix in- literally means "to not disturb or agitate." This adjective entered English in the nineteenth century, shortly after the noun form insouciance started being used by English speakers.