(verb)
broadly, to loosely wave or sway back and forth, usually due to having one end fastened and the other end free, and often making a sound/noise in the process;
e.g. The wind made the flags flap on the poles.
(verb)
broadly, to beat at something and/or to use a tool using long strokes, similar to the action involved in flying;
e.g. He started to flap the curtains to get the dust out from them.
(noun)
the action or instance of moving something such as a tool or one's limb (e.g. an arm or a wing) up and down, similar to the action required for flying;
e.g. With a flap of her arms, she signaled to the car to stop at her house.
(noun)
a piece of some thin material like paper or cloth that is secured/fastened on one end and hangs freely on the other, often serving to cover something or protect an opening;
e.g. The coat's pocket has a flap of fabric stitched over it.
(noun)
(informal) a state of panic, agitation, or disruption, often due to excitement;
e.g. The students of the class went into a flap when they heard that they would be taking a surprise quiz.