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]