The Collaborative International Dictionary of English v.0.48:

Extenuate \Ex*ten"u*ate\, v. t. [imp. & p. p. Extenuated; p.
   pr. & vb. n. Extenuating.] [L. extenuatus, p. p. of
   extenuare to make thin, loosen, weaken; ex out + tenuare to
   make thin, tenuis thin. See Tenuity.]
   1. To make thin or slender; to draw out so as to lessen the
      thickness.
      [1913 Webster]

            His body behind the head becomes broad, from whence
            it is again extenuated all the way to the tail.
                                                  --Grew.
      [1913 Webster]

   2. To lessen; to palliate; to lessen or weaken the force of;
      to diminish the conception of, as crime, guilt, faults,
      ills, accusations, etc.; -- opposed to aggravate.
      [1913 Webster]

            But fortune there extenuates the crime. --Dryden.
      [1913 Webster]

            Let us extenuate, conceal, adorn the unpleasing
            reality.                              --I. Taylor.
      [1913 Webster]

   3. To lower or degrade; to detract from. [Obs.]
      [1913 Webster]

            Who can extenuate thee?               --Milton.

   Syn: To palliate; to mitigate. See Palliate.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Extenuate \Ex*ten"u*ate\, v. i.
   To become thinner; to make excuses; to advance palliating
   considerations. --Burke.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Extenuate \Ex*ten"u*ate\, a. [L. extenuatus, p. p.]
   Thin; slender. [Obs.] --Huloet.
   [1913 Webster]

WordNet (r) 3.0 (2006):

extenuate
    v 1: lessen or to try to lessen the seriousness or extent of;
         "The circumstances extenuate the crime" [syn: extenuate,
         palliate, mitigate]

Moby Thesaurus II by Grady Ward, 1.0:

92 Moby Thesaurus words for "extenuate":
   abate, allay, alleviate, allow for, apologize, assuage, attemper,
   attenuate, bank the fire, bate, blunt, chasten, color, constrain,
   control, cramp, cripple, damp, dampen, de-emphasize, deaden,
   debilitate, devitalize, dilute, diminish, downplay, dull, ease,
   enervate, enfeeble, eviscerate, exhaust, explain, gloss over,
   gruel, ignore, justify, keep within bounds, lay, lay low, lenify,
   lessen, lighten, make allowance for, mince, mitigate, moderate,
   modulate, obtund, palliate, play down, qualify, rationalize,
   rattle, reduce, reduce the temperature, remit, restrain, sap,
   shake, shake up, slacken, slow down, slur over, smother, sober,
   sober down, soft-pedal, soften, soften up, stifle, subdue,
   sugarcoat, suppress, tame, temper, tone down, tune down, unbrace,
   undermine, underplay, unman, unnerve, unstrengthen, unstring,
   varnish, veneer, water down, weaken, white, whiten, whitewash