The Collaborative International Dictionary of English v.0.48:

Insouciance \In`sou`ciance"\, n. [F.]
   Carelessness; heedlessness; thoughtlessness; unconcern.
   [1913 Webster]

WordNet (r) 3.0 (2006):

insouciance
    n 1: the cheerful feeling you have when nothing is troubling you
         [syn: carefreeness, insouciance, lightheartedness,
         lightsomeness]