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.