The Collaborative International Dictionary of English v.0.48:

Unkind \Un*kind"\, a.
   1. Not kind; contrary to nature, or the law of kind or
      kindred; unnatural. [Obs.] "Such unkind abominations."
      --Chaucer.
      [1913 Webster]

   2. Wanting in kindness, sympathy, benevolence, gratitude, or
      the like; cruel; harsh; unjust; ungrateful.
      [1913 Webster]

            He is unkind that recompenseth not; but he is most
            unkind that forgetteth.               --Sir T.
                                                  Elyot.
      [1913 Webster] -- Un*kind"ly, adv. -- Un*kind"ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unkindness
    n 1: lack of sympathy [ant: kindness]