Source : Webster's Revised Unabridged Dictionary (1913)

Flap \Flap\, v. t. [imp. & p. p. {Flapped}; p. pr. & vb. n.
   {Flapping}.] [Prob. of imitative origin; cf. D. flappen, E.
   flap, n., flop, flippant, fillip.]
   1. To beat with a flap; to strike.

            Yet let me flap this bug with gilded wings. --Pope.

   2. To move, as something broad and flaplike; as, to flap the
      wings; to let fall, as the brim of a hat.

   {To flap in the mouth}, to taunt. [Obs.] --W. Cartwright.