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]