The Collaborative International Dictionary of English v.0.48:

Weaken \Weak"en\, v. t. [imp. & p. p. Weakened; p. pr. & vb.
   n. Weakening.]
   [1913 Webster]
   1. To make weak; to lessen the strength of; to deprive of
      strength; to debilitate; to enfeeble; to enervate; as, to
      weaken the body or the mind; to weaken the hands of a
      magistrate; to weaken the force of an objection or an
      argument.
      [1913 Webster]

            Their hands shall be weakened from the work, that it
            be not done.                          --Neh. vi. 9.
      [1913 Webster]

   2. To reduce in quality, strength, or spirit; as, to weaken
      tea; to weaken any solution or decoction.
      [1913 Webster]

WordNet (r) 3.0 (2006):

weakened
    adj 1: impaired by diminution [syn: diminished, lessened,
           vitiated, weakened]
    2: made weak or weaker
    3: reduced in strength; "the faded tones of an old recording"
       [syn: attenuate, attenuated, faded, weakened]
    4: mixed with water; "sold cut whiskey"; "a cup of thinned soup"
       [syn: cut, thinned, weakened]
    5: damaged inanimate objects or their value [syn: hurt,
       weakened]

Moby Thesaurus II by Grady Ward, 1.0:

145 Moby Thesaurus words for "weakened":
   abated, ablated, aggravated, attenuated, ausgespielt, bated,
   belittled, broken, burned, burned-out, burst, busted, cachectic,
   checked, chipped, consumed, contracted, cracked, crazed, curtailed,
   cut, damaged, debilitated, decreased, deflated, deteriorated,
   devitalized, diminished, disabled, dissipated, drained, drooping,
   droopy, dropped, effete, embittered, enervated, enfeebled, eroded,
   eviscerated, exacerbated, exhausted, fagged, failing, faint,
   fainting, fallen, fatigued, feeble, feeling faint, flagging,
   footsore, frail, frazzled, good and tired, harmed, healthless,
   hurt, impaired, imperfect, in bits, in pieces, in poor health,
   in shards, incapacitated, infirm, injured, invalid, irritated,
   jaded, lacerated, languid, languishing, less, lesser, lower,
   lowered, mangled, miniaturized, moribund, mutilated, pale, peaked,
   peaky, played out, ready to drop, reduced, reduced in health, rent,
   retrenched, run ragged, run-down, ruptured, sagging, sapped,
   scalded, scaled-down, scorched, seedy, shattered, shorn, shorter,
   shrunk, shrunken, sickly, slashed, slit, smaller, smashed, spent,
   split, sprung, the worse for, tired, tired-winged, toilworn, torn,
   unhealthy, unrefreshed, unrestored, unsound, used up,
   valetudinarian, valetudinary, wasted, watered-down, way-weary,
   wayworn, weak, weakly, wearied, weariful, weary, weary-footed,
   weary-laden, weary-winged, weary-worn, wilting,
   with low resistance, worn, worn-down, worn-out, worse, worse off,
   worsened