Source : Webster's Revised Unabridged Dictionary (1913)

Insouciance \In`sou`ciance"\, n. [F.]
   Carelessness; heedlessness; thoughtlessness; unconcern.

Source : WordNet®

insouciance
     n : the cheerful feeling you have when nothing is troubling you
         [syn: {carefreeness}, {lightheartedness}, {lightsomeness}]