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.

Source : WordNet®

weakening
     adj 1: causing debilitation [syn: {debilitative}, {enervating}, {enfeebling}]
     2: moderating by making pain or sorrow weaker
     n 1: becoming weaker [ant: {strengthening}]
     2: the act of reducing the strength of something [ant: {strengthening}]