All you have to do is just keeping your mouth shut.
你只要保守秘密就行了。
keep a secret 保密I can tell you about it, but you have to keep a secret.
我可以告诉你这件事，但你必须保密。
keep to oneself 保守秘密I kept your little secret to myself all these years.
你的这个小秘密我守了这么多年。
button up one's lip 守口如瓶，闭口不言A: Did they find a way at last? 他们最后找到办法了吗?
B: Yes, they worked out a plan. 是的，他们制定出一项计划。
A: I'm most curious about what the plan is. 我很想知道是什么计划。
B: But they buttoned up their lips about it. 可他们对此守口如瓶。
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