The Collaborative International Dictionary of English v.0.48:

Flap \Flap\, n. [OE. flappe, flap, blow, bly-flap; cf. D. flap,
   and E. flap, v.]
   Anything broad and limber that hangs loose, or that is
   attached by one side or end and is easily moved; as, the flap
   of a garment.
   [1913 Webster]

         A cartilaginous flap upon the opening of the larynx.
                                                  --Sir T.
                                                  Browne.
   [1913 Webster]

   2. A hinged leaf, as of a table or shutter.
      [1913 Webster]

   3. The motion of anything broad and loose, or a stroke or
      sound made with it; as, the flap of a sail or of a wing.
      [1913 Webster]

   4. pl. (Far.) A disease in the lips of horses.
      [1913 Webster]

   5. (Aeronautics) a movable part of an airplane wing, used to
      increase lift or drag, especially when taking off or
      landing. used often in the plural.
      [WordNet 1.5]

   Flap tile, a tile with a bent up portion, to turn a corner
      or catch a drip.

   Flap valve (Mech.), a valve which opens and shuts upon one
      hinged side; a clack valve.
      [1913 Webster]