From Middle Englishflap, flappe(“a slap; blow; buffet; fly-flap; something flexible or loose; flap”), related to Saterland FrisianFlappert(“wing, flipper”), Middle Dutchflabbe(“a blow; slap on the face; fly-flap; flap”) (modern Dutchflap(“flap”)), Middle Low Germanflabbe, vlabbe, flebbe, from the verb (see below). Related also to English flab and flabby.
1686, Sir Thomas Browne, chapter VIII, in The Works of the Learned Sr. Thomas Brown: Containing, Enquiries Into Vulgar and Common Errors, etc, book IV:
Again, Beside these parts destin'd to divers offices, there is a peculiar provision for the wind-pipe, that is, a cartilagineous flap upon the opening of the Larynx or Throttle, which hath an open cavity for the admiffion of the air
1998 October, Robert H. Mohlenbrock, “Twin Peaks”, in Natural History, volume 107, number 8, page 73:
The hairs guide the pollinating insect to the base of the petal, where there is a purplish nectary covered by a flap of tissue.
Then he commenced to talk, really talk. and inside of two flaps of a herring's fin he had me mesmerized, like Eben Holt's boy at the town hall show. He talked about the ills of humanity, and the glories of health and Nature and service and land knows what all.
“[…] We saw him vanish right in front of the rest of us. He was there and then he wasn’t. We were to wait for a year for his return or for some message. We waited. Nothing.” / Calvin, his voice cracking: “Jeepers, sir. You must have been in sort of a flap.”
1980 April 19, Mitzel, “Barbre Murder Grand Jury: Puccini Outtake”, in Gay Community News, page 1:
The current Middlesex grand jury […] is once again on the case, partly as the result of the public flap created by Brill's death and, of course, by the series of articles written by Corsetti in the month after Brill's demise.
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