[syn: boring, deadening, dull, ho-hum, irksome, slow, tedious, tiresome, wearisome]
The Collaborative International Dictionary of English v.0.48:
deadening \dead"en*ing\ adj. [p. pr. of verb deaden[3].]
   Rendering less lively, intense, or vigorous; as, the
   deadening effect of some routine tasks.
   [WordNet 1.5]
   2. So lacking in interest as to cause mental weariness; as,
      the deadening effect of some routine tasks.
   Syn: boring, dreary, ho-hum, irksome, tedious, tiresome,
        wearisome.
        [WordNet 1.5]
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.]
The Collaborative International Dictionary of English v.0.48:
deadening \dead"en*ing\ n.
   The act of making something futile and useless (as by
   routine).
   Syn: stultification, impairment.
        [WordNet 1.5]
WordNet (r) 3.0 (2006):
deadening
    adj 1: so lacking in interest as to cause mental weariness; "a
           boring evening with uninteresting people"; "the deadening
           effect of some routine tasks"; "a dull play"; "his
           competent but dull performance"; "a ho-hum speaker who
           couldn't capture their attention"; "what an irksome task
           the writing of long letters is"- Edmund Burke; "tedious
           days on the train"; "the tiresome chirping of a cricket"-
           Mark Twain; "other people's dreams are dreadfully
           wearisome" [syn: boring, deadening, dull, ho-hum,
           irksome, slow, tedious, tiresome, wearisome]
    n 1: the act of making something futile and useless (as by
         routine) [syn: stultification, constipation,
         impairment, deadening]
Moby Thesaurus II by Grady Ward, 1.0:
98 Moby Thesaurus words for "deadening":
   abatement, abating, allaying, allayment, alleviating, alleviation,
   alleviative, analgesia, analgesic, anesthesia, anesthetic,
   anesthetizing, anodyne, appeasement, assuagement, assuaging,
   assuasive, attenuation, attrition, balmy, balsamic, benumbing,
   blunting, calming, cathartic, chastening, cleansing, cushioning,
   dampening, damping, debilitation, demulcent, demulsion,
   devitalization, dilution, diminishing, diminishment, diminution,
   dulcification, dulling, ease, easement, easing, effemination,
   emollient, enervation, enfeeblement, evisceration, exhaustion,
   extenuation, falling-off, fatigue, hushing, inanition,
   languishment, leniency, lenitive, lessening, letdown, letup,
   lightening, loosening, lulling, mitigating, mitigation, mitigative,
   modulation, mollification, narcotic, numbing, pacification,
   pain-killing, palliation, palliative, purgative, quietening,
   quieting, reducing, reduction, relaxation, relaxing, relief,
   relieving, remedial, remedy, remission, salving, slackening,
   softening, soothing, stunning, stupefying, subduement, subduing,
   tempering, thinning, tranquilization, weakening