something broad or flat, flexible or hinged, and usu thin, that hangs loose or covers an opening projects freely, e.g.:
an extended part forming a closure, e.g. of an envelope or carton
a movable control surface on an aircraft wing for increasing lift during takeoff or drag during landing
a flapping movement
the motion of something broad and flexible, e.g. a sail
the up-and-down motion of a wing, esp of a bird
informal a state of confusion or panic
a blow or stroke made using a broad flat object; a slap
flappy adj
[Middle English flappe blow, slap, prob of imitative origin]