The Collaborative International Dictionary of English v.0.48:

moderating \moderating\ adj.
   lessening in intensity or strength. Opposite of
   intensifying. [Narrower terms: tempering; weakening]
   [WordNet 1.5]

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):

weakening
    adj 1: causing debilitation [syn: debilitative, enervating,
           enfeebling, weakening]
    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]