信口开河

xìn kǒu kāi hé

to talk off the top of one’s head

Character by character

信xìn
let

口kǒu
mouth

开kāi
open

河hé
river

没有证据的事，别信口开河。

If you have no proof, don’t just say whatever comes into your head.

Tone: negative (criticism).

Originally 信口开合 (opening and shutting the mouth at will).

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