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]

Moby Thesaurus II by Grady Ward, 1.0:

91 Moby Thesaurus words for "weakening":
   abatement, abridgment, alleviation, attenuation, attrition,
   bankruptcy, blunting, breakage, breakdown, collapse, contraction,
   crack-up, crippling, damage, dampening, damping, deadening,
   debilitation, decrease, decrement, decrescence, deduction,
   deflation, depreciation, depression, destruction, detriment,
   devitalization, devitalizing, dilapidation, dilution, diminishment,
   diminution, disablement, disrepair, draining, dulling, dying,
   dying off, effemination, encroachment, enervating, enervation,
   enfeeblement, enfeebling, evisceration, exhausting, exhaustion,
   extenuation, fade-out, fatigue, fatiguing, grueling, harm,
   hobbling, hurt, hurting, impairment, inanition, incapacitation,
   infringement, injury, inroad, languishment, lessening, letup, loss,
   lowering, maiming, mayhem, miniaturization, mischief, mitigation,
   mutilation, reduction, relaxation, ruination, ruinousness,
   sabotage, sagging, sapping, scaling down, scathe, sickening,
   simplicity, slackening, softening, spoiling, subtraction, thinning,
   trying