The Collaborative International Dictionary of English v.0.48:

Apathetic \Ap`a*thet"ic\, Apathetical \Ap`a*thet"ic*al\a. [See
   Apathy.]
   Void of feeling; not susceptible of deep emotion;
   passionless. a woman who became active rather than apathetic
   as she grew older
   [1913 Webster]

   2. showing a lack of interest or concern; indifferent. an
      apathetic audience

   Syn: indifferent
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

apathetic
    adj 1: showing little or no emotion or animation; "a woman who
           became active rather than apathetic as she grew older"
    2: marked by a lack of interest; "an apathetic audience"; "the
       universe is neither hostile nor friendly; it is simply
       indifferent" [syn: apathetic, indifferent]

Moby Thesaurus II by Grady Ward, 1.0:

173 Moby Thesaurus words for "apathetic":
   Laodicean, Olympian, abeyant, affording no hope, aloof, anesthetic,
   ataractic, backward, balking, balky, benumbed, blah, blase, bleak,
   bored, callous, careless, casual, cataleptic, catatonic, centrist,
   cheerless, comatose, comfortless, dead, debilitated, desensitized,
   despairing, desperate, despondent, detached, devil-may-care,
   dilatory, disconsolate, disinterested, dismal, dispassionate,
   disregardful, distant, dopey, dormant, droopy, drugged, dry, dull,
   easygoing, enervated, even, exanimate, fifty-fifty, flat, forlorn,
   foul, grim, groggy, grudging, half-and-half, heartless, heavy,
   hebetudinous, heedless, hopeless, impartial, impassible, impassive,
   in a stupor, in abeyance, in despair, in suspense, inactive,
   inanimate, inattentive, incurious, independent, indifferent, inert,
   inexcitable, insensible, insensitive, insouciant, jaded,
   lackadaisical, laggard, languid, languorous, latent, leaden,
   lethargic, lifeless, limp, listless, loath, logy, lumpish,
   matter-of-fact, midway, mindless, moderate, moribund, negligent,
   neuter, neutral, nonaligned, nonchalant, noncommitted, nonpartisan,
   numb, numbed, on the fence, passive, perfunctory, phlegmatic,
   pluckless, pococurante, pooped, reckless, regardless, reluctant,
   renitent, resigned, restive, sated, sedentary, slack, sleeping,
   sleepy, slow, slow to, sluggish, slumbering, smoldering, somnolent,
   soporific, spiritless, spunkless, stagnant, stagnating, standing,
   static, stoic, stolid, stultified, stupefied, supine, suspended,
   tame, third-force, third-world, torpid, turned-off, unanxious,
   unaroused, uncaring, uncommitted, unconcerned, undiscriminating,
   unenthusiastic, unhopeful, uninquiring, uninterested, uninvolved,
   unmindful, unmoved, unsolicitous, untouched, unzealous, vegetable,
   vegetative, wan, weary, withdrawn, without hope, world-weary