Definition of flap:
(n) :
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
hinged leaf, as of a table or shutter
"the comment caused quite a flap in the newspapers."
(v) :
to move something broad and loose back and forth, as flap its wings; to move loosely back and forth
"the flag flapped in the breeze."