Source : Webster's Revised Unabridged Dictionary (1913)

Weaken \Weak"en\, v. t. [imp. & p. p. {Weakened}; p. pr. & vb.
   n. {Weakening}.]
   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.

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

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

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.

Source : WordNet®

weaken
     v 1: lessen the strength of; "The fever weakened his body" [ant:
          {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}]
     4: reduce the level or intensity or size or scope of;
        "de-escalate a crisis" [syn: {de-escalate}, {step down}]
        [ant: {escalate}]
     5: lessen in force or effect; "soften a shock"; "break a fall"
        [syn: {dampen}, {damp}, {soften}, {break}]