flap

n. 拍打,拍打声,副翼
v. 拍打,鼓翼而飞,飘动

The curtains were flapping at the open window.

窗帘在敞开的窗户前摆动。

A flap from the tail of the whale upset the boat.

鲸尾轻轻一拍便把小船打翻了。