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]

The Collaborative International Dictionary of English v.0.48:

Weaken \Weak"en\, v. i.
   To become weak or weaker; to lose strength, spirit, or
   determination; to become less positive or resolute; as, the
   patient weakened; the witness weakened on cross-examination.
   "His notion weakens, his discernings are lethargied." --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

weaken
    v 1: lessen the strength of; "The fever weakened his body" [ant:
         beef up, fortify, strengthen]
    2: become weaker; "The prisoner's resistance weakened after
       seven days" [ant: strengthen]
    3: destroy property or hinder normal operations; "The Resistance
       sabotaged railroad operations during the war" [syn:
       sabotage, undermine, countermine, counteract,
       subvert, weaken]
    4: reduce the level or intensity or size or scope of; "de-
       escalate a crisis" [syn: de-escalate, weaken, step
       down] [ant: escalate, intensify, step up]
    5: lessen in force or effect; "soften a shock"; "break a fall"
       [syn: dampen, damp, soften, weaken, break]

Moby Thesaurus II by Grady Ward, 1.0:

220 Moby Thesaurus words for "weaken":
   abate, afflict, aggravate, allay, alleviate, assuage, attemper,
   attenuate, bank the fire, bate, beat, blunt, break, bugger,
   burn out, cave in, chasten, collapse, come apart, come unstuck,
   conk out, constrain, control, cramp, cripple, crumble, cut, damage,
   damp, dampen, de-emphasize, de-energize, deaden, debilitate,
   decline, dematerialize, derange, deteriorate, devitalize,
   dilapidate, dilute, diminish, disable, disedge, disembody,
   disenable, disintegrate, disorder, do in, do up, downplay, drain,
   draw the teeth, droop, drop, dull, dwindle, ease, emacerate,
   emaciate, emasculate, embitter, endamage, enervate, enfeeble,
   etherealize, eviscerate, exacerbate, exhaust, extenuate, fade, fag,
   fag out, fail, faint, fall short, fatigue, fizzle out, flag,
   frazzle, give out, give way, go downhill, go soft, go to pieces,
   gruel, hamstring, harass, harm, hit the skids, honeycomb,
   hors de combat, hospitalize, hurt, impair, inactivate,
   incapacitate, indispose, injure, invalid, irritate, jade,
   keep within bounds, kibosh, knock out, knock up, lame, languish,
   lay, lay low, lay up, lenify, lessen, lighten, lose strength, maim,
   make worse, minimize, mitigate, moderate, modulate, obtund,
   overfatigue, overset, overstrain, overthrow, overtire, overturn,
   overweary, palliate, paralyze, peak, peg out, peter out, pine,
   play down, poop, poop out, prostrate, put, put back, queer,
   queer the works, rarefy, rattle, reduce, reduce the temperature,
   remit, repress, restrain, retund, run down, sabotage, sap,
   sap the foundations, shake, shake up, shrink, sicken, sink,
   slacken, slow down, smother, sober, sober down, soften, soften up,
   spike, spiritualize, stifle, subdue, subtilize, subvert, suppress,
   tame, temper, thin, thin away, thin down, thin out, throw down,
   throw over, tire, tire out, tire to death, tone down, tucker,
   tune down, turn, unbrace, undermine, underplay, unfit, unman,
   unnerve, unstrengthen, unstring, upend, upset, use up, wane, waste,
   waste away, water, water down, wear, wear away, wear down, wear on,
   wear out, wear thin, weary, wilt, wind, wing, wither, wither away,
   worsen, wreck, yield