verb

(of a bird) move (its wings) up and down when flying or preparing to fly.

a pheasant flapped its wings

beat,flutter,move up and down,agitate,wave,wag,waggle,shake,swing,twitch,thresh,thrash,flail,vibrate,quiver,tremble,oscillate

noun

a thin, flat piece of cloth, paper, metal, etc. that is hinged or attached on one side only and covers an opening or hangs down from something.

the flap of the envelope

fold,overhang,overlap,covering,lappet,lap,tab