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.
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.