The Collaborative International Dictionary of English v.0.48:

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.
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            Yet let me flap this bug with gilded wings. --Pope.
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   2. To move, as something broad and flaplike; as, to flap the
      wings; to let fall, as the brim of a hat.
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   To flap in the mouth, to taunt. [Obs.] --W. Cartwright.
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