Something strictly confidential, not to be revealed to a third party; information kept in strict confidence, only between two people.
这个研发项目的核心技术是最高机密，法不传六耳，只能你我二人知晓。
The core technology of this R&D project is top secret; it's a matter strictly between us two, not to be revealed to a third party.
Era:
Early Modern
Frequency:
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