把姓字倒過來

Simplified: 把姓字倒过来

bǎxìngzìdàoguòlái ㄅㄚˇ ㄒㄧㄥˋ ㄗˋ ㄉㄠˋ ㄍㄨㄛˋ ㄌㄞˊ

    • an oath accepting the insult of having one's surname written backwards if one is lying

Character by character

Saying俗语俗語

Idiom惯用语 · Rarely used now

Literally
write one's surname backwards
Meaning
An oath: the speaker accepts the insult of having their surname written backwards if what they say is untrue.

Sources