flap (n.)
mid-14c., flappe "a blow, slap," probably imitative of the sound of striking. Meaning "something that hangs down" is first recorded 1520s. Sense of "motion or noise like a bird's wing" is 1774; meaning "disturbance, noisy tumult" is 1916, British slang.
flap (v.)
early 14c., "dash about, shake;" later "strike, hit;" see flap (n.). Meaning "to swing loosely" is from 1520s. Related: Flapped; flapping.