The Collaborative International Dictionary of English v.0.48:

halfhearted \half"heart`ed\, Half-hearted
\Half"-heart`ed\(-h[aum]rt`[e^]d), a.
   1. Wanting in heart or spirit; ungenerous; unkind. --B.
      Jonson.
      [1913 Webster]

   2. Lacking zeal or courage; performed with less than a full
      effort; lukewarm; unenthusiastic; as, a half-hearted
      attempt; -- of actions. [WordNet sense 1] --H. James.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

halfhearted
    adj 1: feeling or showing little interest or enthusiasm; "a
           halfhearted effort"; "gave only lukewarm support to the
           candidate" [syn: halfhearted, half-hearted, tepid,
           lukewarm]

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "halfhearted":
   Laodicean, bland, changeable, cold, cool, fervorless, indecisive,
   indifferent, infirm of purpose, infirm of will, insipid,
   irresolute, lukewarm, milk-and-water, milky, mushy, neuter,
   neutral, perfunctory, tasteless, tepid, unenthusiastic, vapid,
   watery, wishy-washy, zealless