三缄口

Traditional (Taiwan): 三緘口

sānjiānkǒu ㄙㄢ ㄐㄧㄢ ㄎㄡˇ

    • to be very careful about what one says
    • to keep one's mouth shut

Character by character

Saying俗语俗語

Idiom惯用语 · Rarely used now

Literally
mouth sealed three times
Meaning
To be very careful about what one says and say little; to keep one's mouth shut. Approving or neutral.

Sources