The Collaborative International Dictionary of English v.0.48:

Lethargic \Le*thar"gic\ (l[-e]*th[aum]r"j[i^]k), Lethargical
\Le*thar"gic*al\ (-j[i^]*kal), a. [L. lethargicus, Gr.
   lhqargiko`s: cf. F. l['e]thargique. See Lethargy.]
   Pertaining to, affected with, or resembling, lethargy;
   morbidly drowsy; dull; heavy. -- Le*thar"gic*al*ly, adv. --
   Le*thar"gic*al*ness, n. -- Le*thar"gic*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lethargic
    adj 1: deficient in alertness or activity; "bullfrogs became
           lethargic with the first cold nights" [syn: lethargic,
           unenrgetic] [ant: energetic]

Moby Thesaurus II by Grady Ward, 1.0:

120 Moby Thesaurus words for "lethargic":
   Laodicean, Olympian, aloof, anesthetized, apathetic, benumbed,
   blah, blase, bored, cataleptic, comatose, dead, debilitated,
   desensitized, detached, dilatory, disinterested, doped, dopey,
   dormant, dozy, dreamy, droopy, drowsy, drugged, drugged with sleep,
   dull, enervated, exanimate, exhausted, fagged out, faineant,
   fatigued, half asleep, heartless, heavy, heavy with sleep,
   heavy-eyed, hebetudinous, hopeless, idle, impassive, in a stupor,
   inactive, inanimate, indifferent, indolent, inert, insouciant,
   jaded, lackadaisical, laggard, languid, languorous, lazy, leaden,
   lifeless, listless, lumpish, moribund, napping, narcoleptic,
   narcose, narcotized, narcous, nodding, nonchalant, numb, numbed,
   oscitant, out of it, passive, phlegmatic, pluckless, pooped,
   resigned, sated, sedated, slack, sleep-drowned, sleep-drunk,
   sleep-filled, sleep-swollen, sleepful, sleepy, slothful, slow,
   sluggish, slumberous, slumbery, snoozy, somnolent, soporific,
   spiritless, spunkless, stagnant, stagnating, stoic, stolid,
   stretchy, stultified, stupefied, stupid, stuporose, stuporous,
   supine, tired, torpid, uncaring, unconcerned, uninterested,
   vegetable, vegetative, wan, weak, weary, withdrawn, world-weary,
   yawning, yawny