sut-āu-táu摔後斗

to do the opposite of what one says behind

to do the opposite of what one says behind someone's back

English Wiktionary, “摔後斗” (CC BY-SA 4.0) — https://en.wiktionary.org/wiki/%E6%91%94%E5%BE%8C%E6%96%97 · confidence: Medium (Wiktionary)