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]

Moby Thesaurus II by Grady Ward, 1.0:

103 Moby Thesaurus words for "insouciance":
   abandon, accidia, acedia, aloofness, apathy, ataraxia, ataraxy,
   benumbedness, blah, blahs, boredom, careless abandon, carelessness,
   casualness, comatoseness, cursoriness, detachment, disinterest,
   disinterestedness, dispassion, disregard, disregardfulness,
   dullness, easiness, easygoingness, forgetfulness, hastiness,
   heartlessness, hebetude, heedlessness, hopelessness, impassiveness,
   impassivity, inappetence, inattention, inconsiderateness,
   inconsideration, incuriosity, incuriousness, indifference,
   indifferentism, indifferentness, indiscrimination, inexcitability,
   intellectual inertia, lack of affect, lack of appetite,
   lack of foresight, lack of interest, lackadaisicalness,
   languidness, lassitude, laziness, lethargicalness, lethargy,
   listlessness, mindlessness, negligence, nonchalance, numbness,
   oblivion, offhandedness, passiveness, passivity, perfunctoriness,
   phlegm, phlegmaticalness, phlegmaticness, plucklessness,
   pococurantism, recklessness, regardlessness, resignation,
   resignedness, sloth, sluggishness, sopor, soporifousness,
   spiritlessness, sprezzatura, spunklessness, stolidity,
   stupefaction, stupor, supineness, tactlessness, thoughtlessness,
   torpidity, torpidness, torpor, unanxiousness, unconcern,
   unheedfulness, uninquisitiveness, uninterestedness, unmindfulness,
   unpreparedness, unreadiness, unsolicitousness, unsolicitude,
   unthinkingness, withdrawal, withdrawnness