Insouciance and solicit are secret siblings — both from Latin "thoroughly stirred up," but insouciance means refusing to be stirred at all.
A casual lack of concern; lighthearted indifference or unconcern.
From French insouciance (carelessness, unconcern), from insouciant (carefree, unconcerned), from in- (not) + souciant (caring, worrying), present participle of soucier (to worry, to trouble), from Latin sollicitāre (to disturb, to agitate), from sollicitus (thoroughly stirred up). Key roots: sollicitus (Latin: "thoroughly stirred up (from sollus, whole + citus, moved)").
Insouciance shares a hidden root with solicit — both trace to Latin sollicitus (thoroughly stirred up, agitated). To solicit is to stir someone up to action; insouciance is the refusal to be stirred. The word is distinctly French in character and was borrowed precisely because English lacked a word that captured this specific quality — not mere carelessness (which implies negligence) but a graceful, confident lack of worry. French culture elevated insouciance to an aesthetic ideal,