Stunning, rude dismissal (4,4)
I believe the answer is:
drop dead
If this is the answer I cannot explain how the clue works.
Can you help me to learn more?
(Other definitions for drop dead that I've seen before include "Absolutely (gorgeous)" , "Way to go!" , "Rude injunction to fall lifeless" , "Really offensive remark" , "to appear so gorgeous" .)