FLAP, noun [Latin alapa, a slap. It seems difficult to separate flap from clap, slap, flabby, lap, etc.]
1. Any thing broad and limber that hangs loose, or is easily moved.
A cartilaginous flap on the opening of the larynx.
We say, the flap of a garment, the flap of the ear, the flap of a hat.
2. The motion of any thing broad and loose, or a stroke with it.
3. The flaps, a disease in the lips of horses.
FLAP, verb transitive
1. To beat with a flap
Yet let me flap this bug with gilded wings.
2. To move something broad; as, to flap the wings.
3. To let fall, as the brim of a hat. [This sense seems to indicate a connection with lap.]
FLAP, verb intransitive
1. To move as wings, or as something broad or loose.
2. To fall, as the brim of a hat, or other broad thing.