Slang: Hush-hush

Definition: Very secret.

Example:

1) The operation was so hush-hush that even the commanding officer didn't know all of the details.

2) Let's keep that information hush-hush, OK?

Etymology: 'Hush' means 'quiet'. So something that is 'hush-hush' should not be discussed or exposed in public. This phrase dates back to Word War I, when it referred to military secrets. 

fonte: www.peakenglish.com