in·sou·ci·ance /ɪnˈsusiən(t)s, ænsusjɑns/
  漫不經心;無憂無慮
  In·sou·ciance n.  Carelessness; heedlessness; thoughtlessness; unconcern.
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  insouciance
       n : the cheerful feeling you have when nothing is troubling you
           [syn: carefreeness, lightheartedness, lightsomeness]