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.
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   2. Slow; having little motion; as, a sluggish stream.
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   3. Having no power to move one's self or itself; inert.
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            Matter, being impotent, sluggish, and inactive, hath
            no power to stir or move itself.      --Woodward.
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            And the sluggish land slumbers in utter neglect.
                                                  --Longfellow.
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   4. Characteristic of a sluggard; dull; stupid; tame; simple.
      [R.] "So sluggish a conceit." --Milton.
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   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):

sluggishness
    n 1: a state of comatose torpor (as found in sleeping sickness)
         [syn: lethargy, lassitude, sluggishness]
    2: the pace of things that move relatively slowly; "the
       sluggishness of the economy"; "the sluggishness of the
       compass in the Arctic cold"
    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:

19 Moby Thesaurus words for "sluggishness":
   hebetude, idleness, indolence, laggardness, languidness, languor,
   lassitude, laze, laziness, lethargy, lifelessness, phlegm,
   pococurantism, shiftlessness, sloth, slothfulness, slouch,
   stagnation, torpor