The Collaborative International Dictionary of English v.0.48:

Deaden \Dead"en\ (d[e^]d"'n), v. t. [imp. & p. p. Deadened
   (d[e^]d"'nd); p. pr. & vb. n. Deadening.] [From Dead; cf.
   AS. d?dan to kill, put to death. See Dead, a.]
   1. To make as dead; to impair in vigor, force, activity, or
      sensation; to lessen the force or acuteness of; to blunt;
      as, to deaden the natural powers or feelings; to deaden a
      sound.
      [1913 Webster]

            As harper lays his open palm
            Upon his harp, to deaden its vibrations.
                                                  --Longfellow.
      [1913 Webster]

   2. To lessen the velocity or momentum of; to retard; as, to
      deaden a ship's headway.
      [1913 Webster]

   3. To make vapid or spiritless; as, to deaden wine.
      [1913 Webster]

   4. To deprive of gloss or brilliancy; to obscure; as, to
      deaden gilding by a coat of size.
      [1913 Webster]

   5. To render impervious to sound, as a wall or floor; to
      deafen.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

deaden
    v 1: make vague or obscure or make (an image) less visible;
         "muffle the message" [syn: dampen, deaden, damp]
    2: cut a girdle around so as to kill by interrupting the
       circulation of water and nutrients; "girdle the plant" [syn:
       girdle, deaden]
    3: make vapid or deprive of spirit; "deadened wine"
    4: lessen the momentum or velocity of; "deaden a ship's headway"
    5: become lifeless, less lively, intense, or active; lose life,
       force, or vigor
    6: make less lively, intense, or vigorous; impair in vigor,
       force, activity, or sensation; "Terror blunted her feelings";
       "deaden a sound" [syn: deaden, blunt] [ant: animate,
       enliven, invigorate, liven, liven up]
    7: convert (metallic mercury) into a grey powder consisting of
       minute globules, as by shaking with chalk or fatty oil

Moby Thesaurus II by Grady Ward, 1.0:

127 Moby Thesaurus words for "deaden":
   KO, abate, absorb the shock, allay, alleviate, anesthetize,
   appease, assuage, attemper, attenuate, baffle, bank the fire,
   bedaze, benumb, besot, blunt, break the fall, chasten, chloroform,
   coldcock, constrain, control, cramp, cripple, cushion, damp,
   dampen, de-emphasize, deaden the pain, deafen, debilitate,
   desensitize, devitalize, diminish, dope, downplay, drug, dull,
   ease, ease matters, enervate, enfeeble, etherize, eviscerate,
   exhaust, extenuate, foment, freeze, give relief, gruel, kayo,
   keep within bounds, knock out, knock senseless, knock stiff,
   knock unconscious, lay, lay low, lay out, lenify, lessen, lighten,
   lull, mat, mitigate, moderate, modulate, mollify, muffle, mull,
   mute, narcotize, neutralize, numb, obtund, offset, pad, palliate,
   palsy, paralyze, play down, poultice, pour balm into, pour oil on,
   put to sleep, rattle, reduce, reduce the temperature, relieve,
   restrain, salve, sap, shake, shake up, show consideration,
   show mercy, show pity, slacken, slake, slow down, smother, sober,
   sober down, soft-pedal, soften, soften the blow, soften up, soothe,
   stifle, stop, stun, stupe, stupefy, subdue, suppress, tame, temper,
   tone down, tune down, unbrace, undermine, underplay, unman,
   unnerve, unstrengthen, unstring, weaken