Source : Webster's Revised Unabridged Dictionary (1913)

Damp \Damp\, a. [Compar. {Damper}; superl. {Dampest}.]
   1. Being in a state between dry and wet; moderately wet;
      moist; humid.

            O'erspread with a damp sweat and holy fear.
                                                  --Dryden.

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

            All these and more came flocking, but with looks
            Downcast and damp.                    --Milton.

Damper \Damp"er\, 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.

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

Source : WordNet®

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:
        {muffler}]
     3: a depressing restraint; "rain put a damper on our picnic
        plans"