Flap, n.
Etymology: [OE. flappe, flap, blow, bly-flap; cf. D. flap, and E. flap, v.]
Anything broad and limber that hangs loose, or that is attached by one side or end and is easily moved; as, the flap of a garment. A cartilaginous flap upon the opening of the larynx. Sir T. Browne.
2. A hinged leaf, as of a table or shutter.
3. The motion of anything broad and loose, or a stroke or sound made with it; as, the flap of a sail or of a wing.
4. pl. (Far.)
1 more senses
A disease in the lips of horses. Flap tile, a tile with a bent up portion, to turn a corner or catch a drip. -- Flap valve (Mech.), a valve which opens and shuts upon one hinged side; a clack valve.
2 other Webster entries under this spelling
Flap, v. t. [imp. & p. p. Flapped; p. pr. & vb. n. Flapping.]
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.
Flap, v. i.
1. To move as do wings, or as something broad or loose; to fly with wings beating the air. The crows flapped over by twos and threes. Lowell.
2. To fall and hang like a flap, as the brim of a hat, or other broad thing. Gay.
Shades of meaning
Fernald has no entry of its own for flap; he lists it under shake.
Where he writes about it:
Flap, flutter, and fluctuate refer to wave-like movements, flap generally to such as produce a sharp sound; a cock flaps his wings; flutter applies to a less pronounced and more irregular motion; a captive bird or a feeble pulse flutters. — from shake