The Cone of Silence is one of many recurring joke devices from Get Smart, a 1960s American comedy television series about an inept spy. The essence of the joke is that the apparatus, designed for secret conversations, makes it impossible for those inside the device – and easy for those outside the device – to hear the conversation. The end result being neither secret nor communication.
In popular culture, "cone of silence" is a slang phrase meaning that the speaker wishes to keep the indicated information secret and that the conversation should not be repeated to anyone not currently present. For example: "We aren't inviting Steve to come geocaching with us as he just whines all the time, but keep that in the cone of silence."
