cone of silence 

An imaginary zone from which sound or discussion can't escape. The cone of silence is used to keep something quiet or private. Most often applied to spoken communication, but can also apply to writing or email.

From a running gag on the TV show "Get Smart" in which a plexiglass device would lower from the ceiling and cover the heads of two speakers, so that their conversation couldn't be spied upon. The cone of silence rarely worked correctly on TV, and usually isn't much better in real life.

Employee 1: Hey, I just overheard your boss talking about you. You won't believe what he said.

Employee 2: Let's take this one to the cone of silence.

cone of silence by Evac156 May 2, 2005

cone of silence 

Slang phrase meaning meaning that you want to keep the indicated information secret so it shouldn't be repeated to anyone not currently present

Keep it in the cone of silence, we aren't inviting Cindy and her boyfriend to the movies because they embarrass us.

cone of silence by Vulgarmagick January 18, 2015