The Collaborative International Dictionary of English v.0.48:

Inert \In*ert"\, a. [L. iners, inertis, unskilled, idle; pref.
   in- + ars art: cf. F. inerte. See Art.]
   [1913 Webster]
   1. Destitute of the power of moving itself, or of active
      resistance to motion; as, matter is inert.
      [1913 Webster]

   2. Indisposed to move or act; very slow to act; sluggish;
      dull; inactive; indolent; lifeless.
      [1913 Webster]

            The inert and desponding party of the court.
                                                  --Macaulay.
      [1913 Webster]

            It present becomes extravagant, then imbecile, and
            at length utterly inert.              --I. Taylor.
      [1913 Webster]

   3. Not having or manifesting active properties; not affecting
      other substances when brought in contact with them;
      powerless for an expected or desired effect; as, the noble
      gases are chemically inert.

   Syn: Inactive; dull; passive; indolent; sluggish; slothful;
        lazy; lifeless; irresolute; stupid; senseless;
        insensible.

   Usage: Inert, Inactive, Sluggish. A man may be inactive
          from mere lack of stimulus to effort; but one who is
          inert has something in his constitution or his habits
          which operates like a weight holding him back from
          exertion. Sluggish is still stronger, implying some
          defect of temperament which directly impedes action.
          Inert and inactive are negative, sluggish is positive.
          [1913 Webster]

                Even the favored isles . . .
                Can boast but little virtue; and, inert
                Through plenty, lose in morals what they gain
                In manners -- victims of luxurious ease.
                                                  --Cowper.
          [1913 Webster]

                Doomed to lose four months in inactive
                obscurity.                        --Johnson.
          [1913 Webster]

                Sluggish Idleness, the nurse of sin,
                Upon a slothful ass he chose to ride. --Spenser.
          [1913 Webster]

WordNet (r) 3.0 (2006):

inert
    adj 1: unable to move or resist motion
    2: having only a limited ability to react chemically; chemically
       inactive; "inert matter"; "an indifferent chemical in a
       reaction" [syn: inert, indifferent, neutral]
    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:

144 Moby Thesaurus words for "inert":
   abeyant, abiotic, apathetic, asleep, azoic, benumbed, blase, bored,
   cataleptic, catatonic, changeless, constant, contemplative, dead,
   debilitated, do-nothing, dopey, dormant, droopy, drugged, dull,
   dumb, enervated, exanimate, flat, foul, groggy, heavy,
   hebetudinous, idle, immobile, immutable, impassive, impotent,
   in abeyance, in suspense, inactive, inanimate, inanimated,
   incommutable, inconvertible, indefeasible, indolent, insensate,
   insensible, insentient, insusceptible of change, intransmutable,
   invariable, irretrievable, irreversible, irrevocable, jaded,
   lackadaisical, laissez-aller, laissez-faire, languid, languorous,
   lasting, latent, lazy, leaden, lethargic, lifeless, listless, logy,
   lumpish, meditative, moribund, motionless, mute, neuter, neutral,
   noble, nonconscious, nonliving, nonreturnable, nonreversible, numb,
   otiose, paralytic, paralyzed, passive, permanent, phlegmatic,
   pooped, powerless, procrastinating, quiescent, quiet, quietist,
   quietistic, reverseless, sated, sedentary, senseless, slack,
   sleeping, sleepy, slothful, slow, sluggish, slumbering, smoldering,
   somnolent, soulless, stagnant, stagnating, standing, standpat,
   static, stationary, still, stolid, stultified, supine, suspended,
   tame, torpid, unalterable, unalterative, unaltered, unanimated,
   unaroused, unchangeable, unchanged, unchanging, unconscious,
   undeflectable, undeviating, unfeeling, unmodifiable, unremitting,
   unresponsive, unrestorable, unreturnable, unsusceptible,
   unvariable, unvarying, vegetable, vegetative, wan, weary,
   world-weary