Ah, a little bit more of a colloquial 成语 for y’all today.
shǒukǒurúpíng literally means “to guard one’s mouth like a closed bottle.” Can you imagine what it means?
If you said, perhaps, “to keep your mouth shut” or “to not breathe a word about it” you were right! If you mimed zipping your mouth shut, you are also correct! It’s nice and flexible, so I’ll go through some of the possible uses.
守口如瓶的人 means “a tight-lipped person” or someone apt at keeping secrets, or general information to themselves. So here it’s more of an adjective.
对 [some topic] 守口如瓶 means “to be tight-lipped or secretive about [some topic]” For example, 她对她家庭的事守口如瓶。 This can be translated as “She keeps quiet about her family’s business.”
Finally, if we want to just tell someone to keep their mouth shut, we can say to them, 对这件事要守口如瓶。
Alright folks, short post, but I think you should be able to 保密 [bǎomì] or “keep a secret” or even tell someone to keep a secret (a secret, I should mention is a 秘密 [mìmì], perhaps the most delightful word to say!)