The Collaborative International Dictionary of English v.0.48:

Unconcern \Un`con*cern"\, n.
   Want of concern; absence of anxiety; freedom from solicitude;
   indifference.
   [1913 Webster]

         A listless unconcern,
         Cold, and averting from our neighbor's good. --Thomson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unconcern
    n 1: the trait of remaining calm and seeming not to care; a
         casual lack of concern [syn: nonchalance, unconcern,
         indifference]
    2: a feeling of lack of concern [ant: concern]

Moby Thesaurus II by Grady Ward, 1.0:

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