Insouciance is a feeling of careless indifference. There's a certain amount of lightheartedness in insouciance, but rather than merely being cheerful, someone with insouciance truly couldn't care less.
Insouciance has roots in the French in, meaning "not," and se soucier, meaning "to care," giving the English word its "uncaring" meaning. Insouciance can be a positive state — like the childlike insouciance you feel when you are watching cartoons instead of paying your bills. However, insouciance is not always so sunny. Young voters are often accused of treating the right to vote with insouciance, meaning they just can't be bothered.