The Collaborative International Dictionary of English v.0.48:

Lethargy \Leth"ar*gy\ (-j[y^]), n.; pl. -gies (-j[i^]z). [F.
   l['e]thargie, L. lethargia, Gr. lhqargi`a, fr. lh`qargos
   forgetful, fr. lh`qh forgetfulness. See Lethe.]
   1. Morbid drowsiness; continued or profound sleep, from which
      a person can scarcely be awaked.
      [1913 Webster]

   2. A state of inaction or indifference.
      [1913 Webster]

            Europe lay then under a deep lethargy. --Atterbury.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Lethargy \Leth"ar*gy\, v. t.
   To lethargize. [Obs.] --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lethargy
    n 1: a state of comatose torpor (as found in sleeping sickness)
         [syn: lethargy, lassitude, sluggishness]
    2: weakness characterized by a lack of vitality or energy [syn:
       inanition, lassitude, lethargy, slackness]
    3: inactivity; showing an unusual lack of energy; "the general
       appearance of sluggishness alarmed his friends" [syn:
       languor, lethargy, sluggishness, phlegm, flatness]

Moby Thesaurus II by Grady Ward, 1.0:

176 Moby Thesaurus words for "lethargy":
   abstraction, abulia, acedia, alienation, aloofness, anxiety,
   anxiety equivalent, anxiety state, apathy, asininity, ataraxia,
   ataraxy, benumbedness, blah, blahs, blockishness, boobishness,
   boorishness, boredom, bovinity, catalepsy, catatonic stupor,
   cloddishness, coma, comatoseness, compulsion, crassness, crudeness,
   dejection, denseness, density, depression, detachment,
   dim-wittedness, dimness, disinterest, dispassion, disregard,
   dolce far niente, doltishness, doziness, drowsiness,
   dull-wittedness, dullardism, dullness, dumbness, duncery, elation,
   emotionalism, encephalitis lethargica, enervation, ennui, euphoria,
   exhaustion, fatigue, folie du doute, grossness, heartlessness,
   heaviness, hebetude, heedlessness, high, hopelessness,
   hypochondria, hysteria, hysterics, idleness, impassivity,
   inactivity, inanimation, inanition, inappetence, indifference,
   indolence, inertia, inertness, insensibility, insouciance,
   jadedness, lack of appetite, lackadaisicalness, languidness,
   languishment, languor, languorousness, lassitude, laziness,
   lenitude, lentor, lethargicalness, lifelessness, listlessness,
   loutishness, lumpishness, mania, melancholia, mental distress,
   narcohypnosis, narcolepsy, narcoma, narcosis, narcotic stupor,
   narcotization, ninnyism, nod, nonchalance, numbness, oafdom,
   oafishness, obsession, obtuseness, opacity, oscitancy, oscitation,
   pandiculation, passiveness, passivity, pathological indecisiveness,
   phlegm, phlegmaticalness, phlegmaticness, plucklessness,
   preoccupation, psychalgia, psychomotor disturbance, resignation,
   resignedness, satedness, sedation, shock, simpletonianism, sleep,
   sleepiness, sleeping sickness, sloth, slothfulness, slowness,
   sluggishness, slumber, somnolence, somnolency, sopor,
   soporifousness, sottishness, spiritlessness, spunklessness,
   stolidity, stretching, stupefaction, stupidity, stupor, supineness,
   swoon, thanatosis, thick-headedness, thick-wittedness, tic,
   tiredness, torpidity, torpidness, torpitude, torpor, trance,
   twitching, unconcern, unmindfulness, unresponsiveness,
   unteachability, weakness, weariness, withdrawal, withdrawnness,
   world-weariness, wrongheadedness, yawning, yokelism