Flap
Middle English: probably imitative.
wiktionary
From Middle English flap, flappe(“a slap; blow; buffet; fly-flap; something flexible or loose; flap”), related to Middle Dutch flabbe(“a blow; slap on the face; fly-flap; flap”) (modern Dutch flap(“flap”)), Middle Low German flabbe, vlabbe, flebbe, from the verb (see below). Related also to English flab and flabby.
From Middle English flappen(“to flap, clap, slap, strike”), related to Dutch flappen(“to flap”), German Low German flappen(“to flap”), German flappen(“to flap”), Dutch flabberen(“to flit, flap”). Probably ultimately imitative.
etymonline
flap (n.)
mid-14c., flappe "a blow, slap, buffet," probably imitative of the sound of striking. Sense of "device for slapping or striking" is from early 15c. Meaning "something that hangs down" is first recorded 1520s, probably from flap (v.). 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, beat (the wings);" later "strike, hit" (mid-14c.); probably ultimately imitative. Meaning "to swing about loosely" is from 1520s. Related: Flapped; flapping.
