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]

WordNet (r) 3.0 (2006):

listlessness
    n 1: a feeling of lack of interest or energy [syn: languor,
         lassitude, listlessness]
    2: inactivity resulting from lethargy and lack of vigor or
       energy [syn: listlessness, torpidity, torpidness,
       torpor]

Moby Thesaurus II by Grady Ward, 1.0:

125 Moby Thesaurus words for "listlessness":
   accidia, acedia, adynamia, aloofness, anemia, apathy, ataraxia,
   ataraxy, atony, benumbedness, blah, blah feeling, blahs,
   bloodlessness, boredom, cachexia, cachexy, carelessness,
   casualness, comatoseness, cowardice, debilitation, debility,
   detachment, disinterest, disinterestedness, dispassion, disregard,
   disregardfulness, drowsiness, dullness, easygoingness, enervation,
   ennui, etiolation, faintness, fatigue, feebleness, flabbiness,
   flaccidity, heartlessness, heaviness, hebetude, heedlessness,
   hopelessness, impassiveness, impassivity, impotence, inanimation,
   inappetence, inattention, incuriosity, incuriousness, indifference,
   indifferentism, indifferentness, indiscrimination, inexcitability,
   insouciance, intellectual inertia, jadedness, lack of affect,
   lack of appetite, lack of interest, lackadaisicalness, languidness,
   languishment, languor, languorousness, lassitude, lenitude, lentor,
   lethargicalness, lethargy, lifelessness, mindlessness, negligence,
   nonchalance, numbness, oscitancy, passiveness, passivity, phlegm,
   phlegmaticalness, phlegmaticness, plucklessness, pococurantism,
   prostration, recklessness, regardlessness, resignation,
   resignedness, satedness, sleepiness, sloth, slothfulness, slowness,
   sluggishness, softness, somnolence, sopor, soporifousness,
   spiritlessness, spunklessness, stolidity, strengthlessness,
   stupefaction, stupor, supineness, torpidity, torpidness, torpitude,
   torpor, unanxiousness, unconcern, uninquisitiveness,
   uninterestedness, unmindfulness, unsolicitousness, weakliness,
   weakness, weariness, withdrawal, withdrawnness, world-weariness