The Collaborative International Dictionary of English v.0.48:

Sluggish \Slug"gish\, a.
   1. Habitually idle and lazy; slothful; dull; inactive; as, a
      sluggish man.
      [1913 Webster]

   2. Slow; having little motion; as, a sluggish stream.
      [1913 Webster]

   3. Having no power to move one's self or itself; inert.
      [1913 Webster]

            Matter, being impotent, sluggish, and inactive, hath
            no power to stir or move itself.      --Woodward.
      [1913 Webster]

            And the sluggish land slumbers in utter neglect.
                                                  --Longfellow.
      [1913 Webster]

   4. Characteristic of a sluggard; dull; stupid; tame; simple.
      [R.] "So sluggish a conceit." --Milton.
      [1913 Webster]

   Syn: Inert; idle; lazy; slothful; indolent; dronish; slow;
        dull; drowsy; inactive. See Inert.
        [1913 Webster] -- Slug"gish*ly, adv. --
        Slug"gish*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

sluggish
    adj 1: moving slowly; "a sluggish stream" [syn: sluggish,
           sulky]
    2: (of business) not active or brisk; "business is dull (or
       slow)"; "a sluggish market" [syn: dull, slow, sluggish]
    3: slow and apathetic; "she was fat and inert"; "a sluggish
       worker"; "a mind grown torpid in old age" [syn: inert,
       sluggish, soggy, torpid]

Moby Thesaurus II by Grady Ward, 1.0:

215 Moby Thesaurus words for "sluggish":
   Laodicean, Micawberish, Olympian, abeyant, affluent, aloof,
   ambling, apathetic, backward, benumbed, blah, blase, blunt,
   blunt-witted, bored, cataleptic, catatonic, cautious, circumspect,
   claudicant, comatose, confluent, costive, coursing, crawling,
   creeping, creeping like snail, dallying, dead, debilitated,
   decurrent, defluent, delaying, deliberate, desensitized, detached,
   diffluent, dilatory, dillydallying, dim, dim-witted, disinterested,
   dopey, dormant, down, dragging, draggy, droopy, drugged, dull,
   dull of mind, dull-headed, dull-pated, dull-witted, easy,
   easygoing, enervated, exanimate, faltering, fat-witted, flagging,
   flat, flowing, fluent, fluxional, fluxive, foot-dragging, foul,
   gentle, gradual, groggy, gross-headed, gulfy, gushing, halting,
   heartless, heavy, hebetudinous, hobbled, hobbling, hopeless, idle,
   in a stupor, in abeyance, in suspense, inactive, inanimate,
   indifferent, indolent, inert, insouciant, jaded, lackadaisical,
   laggard, lagging, languid, languorous, latent, lax, lazy, leaden,
   leisurely, lethargic, lifeless, limping, lingering, listless, logy,
   loitering, lumbering, lumpish, mazy, meandering, moderate,
   moribund, nonchalant, numb, numbed, obtuse, off, passive,
   phlegmatic, pluckless, poking, poky, pooped, pouring,
   procrastinating, procrastinative, procrastinatory, profluent,
   racing, relaxed, reluctant, remiss, resigned, running, rushing,
   sated, sauntering, sedentary, serpentine, shuffling, slack,
   sleeping, sleepy, slothful, slow, slow as death, slow as molasses,
   slow as slow, slow-crawling, slow-foot, slow-going, slow-legged,
   slow-moving, slow-paced, slow-poky, slow-running, slow-sailing,
   slow-stepped, slow-witted, slumbering, slumberous, smoldering,
   snail-paced, snaillike, somnolent, soporific, spiritless,
   spunkless, staggering, stagnant, stagnating, standing, static,
   stiff, stoic, streaming, strolling, stultified, stupefied, stupid,
   supine, surging, surgy, suspended, tame, tentative, thick-brained,
   thick-headed, thick-pated, thick-witted, thickskulled, tidal,
   toddling, torpid, tortoiselike, tottering, trudging, turtlelike,
   unaroused, uncaring, unconcerned, unhurried, uninterested,
   vegetable, vegetative, vortical, waddling, wan, weary, withdrawn,
   wooden, world-weary