To flap gracefully, flap (like a flag).

Sleeves are sleeves.

Dress refers to clothes, especially dresses.

Flying sleeve is a special kind of sleeve, which is characterized by elegance. Designers express their gentle and graceful side to women through flying sleeves.

You can search for "flying sleeves" on Baidu pictures, and you will see various flying sleeve designs.

(These are clothes that obviously use the flying sleeve design. )