Source : Webster's Revised Unabridged Dictionary (1913)

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

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

Source : WordNet®

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