The Collaborative International Dictionary of English v.0.48:

Dampen \Damp"en\ (d[a^]mp"'n), v. t. [imp. & p. p. dampened
   (d[a^]mp"'nd); p. pr. & vb. n. dampening.]
   1. To make damp or moist; to make slightly wet.
      [1913 Webster]

   2. To depress; to check; to make dull; to lessen.
      [1913 Webster]

            In a way that considerably dampened our enthusiasm.
                                                  --The Century.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Dampen \Damp"en\, v. i.
   To become damp; to deaden. --Byron.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dampen
    v 1: smother or suppress; "Stifle your curiosity" [syn:
         stifle, dampen] [ant: excite, stimulate]
    2: make moist; "The dew moistened the meadows" [syn: moisten,
       wash, dampen]
    3: deaden (a sound or noise), especially by wrapping [syn:
       muffle, mute, dull, damp, dampen, tone down]
    4: reduce the amplitude (of oscillations or waves)
    5: make vague or obscure or make (an image) less visible;
       "muffle the message" [syn: dampen, deaden, damp]
    6: check; keep in check (a fire)
    7: lessen in force or effect; "soften a shock"; "break a fall"
       [syn: dampen, damp, soften, weaken, break]

Moby Thesaurus II by Grady Ward, 1.0:

198 Moby Thesaurus words for "dampen":
   abate, abridge, absorb the shock, allay, alleviate, arrest,
   asperge, assuage, attemper, attenuate, baffle, bank the fire,
   beat down, bedew, bespatter, besprinkle, blunt, bottle up,
   break the fall, cast down, chasten, check, chill, compress,
   constrain, control, cool, countercheck, cramp, cripple, curb,
   curtail, cushion, cut, cut back, cut down, dabble, dam up, damp,
   dampen the spirits, darken, dash, de-emphasize, deaden, deafen,
   debilitate, decrease, deduct, deflate, deflect, deject, delay,
   depreciate, depress, detain, deter, devitalize, dew, diminish,
   disaffect, discourage, dishearten, disincline, disinterest,
   dispirit, distract, divert, douche, downgrade, downplay, dull,
   enervate, enfeeble, eviscerate, exhaust, extenuate, gruel, hinder,
   hold back, hold in check, hold up, hose, hose down, humect,
   humectate, humidify, impede, indispose, inhibit, intercept,
   interfere, intermeddle, interrupt, intervene, irrigate, keep back,
   keep in check, keep within bounds, knock down, lay, lay low,
   lenify, lessen, lighten, lower, lower the spirits, meddle,
   mitigate, moderate, modulate, moisten, muffle, mute, neutralize,
   obtund, offset, oppose, oppress, paddle, palliate, pare, play down,
   press down, put off, quench, rattle, reduce,
   reduce the temperature, repel, repress, resist, restrain, retard,
   retrench, roll back, sadden, sap, scale down, scotch, set back,
   shake, shake up, shorten, show consideration, show mercy,
   show pity, simplify, sink, slacken, slobber, slop, slosh,
   slow down, smother, snub, sober, sober down, soft-pedal, soften,
   soften the blow, soften up, sparge, spatter, splash, splatter,
   sponge, spray, sprinkle, step down, stifle, stop, subdue, suppress,
   swash, syringe, take from, tame, temper, tone down, tune down,
   turn aside, turn away, turn from, turn off, unbrace, undermine,
   underplay, unman, unnerve, unstrengthen, unstring, water, weaken,
   wean from, weigh heavy upon, weigh upon, wet, wet down