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]