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"