The Collaborative International Dictionary of English v.0.48:

Listless \List"less\, a. [OE. listles, lustles. See Lust.]
   Having no desire or inclination; indifferent; heedless;
   spiritless. " A listless unconcern." --Thomson.
   [1913 Webster]

         Benumbed with cold, and listless of their gain.
                                                  --Dryden.
   [1913 Webster]

         I was listless, and desponding.          --Swift.

   Syn: Heedless; careless; indifferent; vacant; uninterested;
        languid; spiritless; supine; indolent. --
        List"less*ly, adv. -- List"less*ness, n.
        [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

143 Moby Thesaurus words for "listless":
   Laodicean, Olympian, aloof, anemic, apathetic, asthenic, ataractic,
   benumbed, blah, blase, bloodless, bored, careless, casual, chicken,
   comatose, cool, cowardly, dead, debilitated, desensitized,
   detached, devil-may-care, disinterested, dispassionate,
   disregardful, distant, dopey, dormant, drooping, droopy, drugged,
   dull, easygoing, effete, enervated, etiolated, exanimate, faint,
   faintish, feeble, flabby, flaccid, floppy, gone, gutless,
   heartless, heavy, hebetudinous, heedless, hopeless, imbecile,
   impassive, impotent, in a stupor, inanimate, inattentive,
   incurious, indifferent, inert, inexcitable, insouciant, jaded,
   lackadaisical, languid, languishing, languorous, leaden, lethargic,
   lifeless, limber, limp, lukewarm, lumpish, lustless, marrowless,
   mindless, moribund, negligent, nerveless, nonchalant, numb, numbed,
   passive, perfunctory, phlegmatic, pithless, pluckless, pococurante,
   pooped, powerless, reckless, regardless, resigned, rubbery,
   sapless, sated, sinewless, slack, sleepy, slow, sluggish, soft,
   somnolent, soporific, spent, spineless, spiritless, spunkless,
   stagnant, stagnating, stoic, stolid, strengthless, stultified,
   stupefied, supine, tepid, thoughtless, torpid, turned-off,
   unanxious, uncaring, unconcerned, undiscriminating, unemotional,
   unenthusiastic, unhardened, uninquiring, uninterested, uninvolved,
   unmindful, unnerved, unsolicitous, unstrung, vegetable, vegetative,
   wan, weak, weakly, weary, withdrawn, world-weary