Top secret
Definition 1 of 1
Pro Tip 1/3Use in Trust
Apply 'top secret' when discussing matters that require utmost trust and discretion. 
He shared the top secret strategy only with his closest advisors.
Pro Tip 2/3Government Context
'Top secret' is often used in relation to government or military information not for the public. 
The documents were labeled 'top secret' and kept under lock and key.
Pro Tip 3/3Drama and Intensity
Using 'top secret' can add drama or seriousness to a situation, even humorously in casual talk. 
My surprise party plans are top secret, so don't tell the birthday girl!