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]