If something is hush-hush, it is meant to stay very secret. The people who know are not supposed to talk. You will hear it about a work project, a surprise party, a government plan, sometimes a family matter. A hush-hush meeting. They kept it hush-hush. The word is informal and a little playful — almost the sound of “shh” said twice. Newspapers use it with a wink. A lawyer’s letter would say confidential. Though actually it is not the same as Hush! in the moment. Hush! means be quiet now. Hush-hush means this whole thing must not get out. It can sit before a noun or after a verb. Slightly old-fashioned, but people still say it, often with a small smile.