The Collaborative International Dictionary of English v.0.48:

Unkind \Un*kind"\, a. [See Kin kindred.]
   Having no race or kindred; childless. [Obs. & R.] --Shak.
   [1913 Webster]

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):

unkind
    adj 1: lacking kindness; "a thoughtless and unkind remark"; "the
           unkindest cut of all" [ant: kind]
    2: deficient in humane and kindly feelings [syn: pitiless,
       unkind]

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "unkind":
   callous, cruel, disagreeable, flinty, hard, harsh, heartless, ill,
   inconsiderate, inflexible, inhospitable, inhuman, insensitive,
   mean, rigid, severe, stern, thoughtless, tough, unaffectionate,
   unamiable, unbending, unbenign, unbenignant, uncaring,
   uncharitable, unchristian, uncompassionate, uncompassioned,
   unconcerned, uncordial, unfeeling, ungenial, ungracious, unkindly,
   unloving, unsympathetic, unsympathizing, unthoughtful,
   unyielding