The Collaborative International Dictionary of English v.0.48:

Damp \Damp\ (d[a^]mp), a. [Compar. Damper; superl. Dampest.]
   1. Being in a state between dry and wet; moderately wet;
      moist; humid.
      [1913 Webster]

            O'erspread with a damp sweat and holy fear.
                                                  --Dryden.
      [1913 Webster]

   2. Dejected; depressed; sunk. [R.]
      [1913 Webster]

            All these and more came flocking, but with looks
            Downcast and damp.                    --Milton.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

damper \damp"er\ (d[a^]m"p[~e]r), n.
   That which damps or checks; as:
   (a) A valve or movable plate in the flue or other part of a
       stove, furnace, etc., used to check or regulate the
       draught of air.
   (b) A contrivance, as in a pianoforte, to deaden vibrations;
       or, as in other pieces of mechanism, to check some action
       at a particular time.
       [1913 Webster]

             Nor did Sabrina's presence seem to act as any
             damper at the modest little festivities. --W.
                                                  Black.
       [1913 Webster]

WordNet (r) 3.0 (2006):

damper
    n 1: a movable iron plate that regulates the draft in a stove or
         chimney or furnace
    2: a device that decreases the amplitude of electronic,
       mechanical, acoustical, or aerodynamic oscillations [syn:
       damper, muffler]
    3: a depressing restraint; "rain put a damper on our picnic
       plans"

Moby Thesaurus II by Grady Ward, 1.0:

86 Moby Thesaurus words for "damper":
   acoustic tile, andiron, antiknock, arrest, baffler, bearing rein,
   bit, brake, chain, check, checkrein, chill, chock, clog,
   coal tongs, cold water, countercheck, crane, crepehanger, crook,
   curb, curb bit, cushion, damp, dampener, determent, deterrent,
   discouragement, doorstop, drag, drag sail, drift anchor,
   drift sail, drogue, fetter, fire hook, fire tongs, firedog, gag,
   grate, grating, grid, griddle, gridiron, grill, griller, holdback,
   hushcloth, killjoy, lifter, martingale, muffle, muffler, mute,
   muzzle, pelham, pessimist, poker, pothook, quietener, remora,
   salamander, scotch, sea anchor, shackle, silence cloth, silencer,
   snaffle, soft pedal, sordine, sordino, sound-absorbing material,
   soundproofing, soundproofing insulation, sourdine, spit,
   spoilsport, spoke, stay, stop, tongs, trammel, tripod, trivet,
   turnspit, wet blanket