top secret
Of the utmost secrecy; not to be revealed to anyone, under any circumstances. Hyphenated if used as before a noun. It should go without saying that this information is top secret—do not mention it to anyone, not even your loved ones. The top-secret memo has been at the center of an ongoing controversy within the party over the past few months.
ˌtop ˈsecret
used to describe very secret government information: These defence plans are top secret, known only to a very few people.The file was marked TOP SECRET.