Sub rosa

What's it mean?  Well, literally "under the rose" but it has taken on the meaning of "in secret".  The idea goes back to the medieval custom of hanging a rose from the ceiling of a room where a meeting was to take place, as a sign that anything said or done there was not to be spoken of outside that room.  You may well see a rose carved above the confessional booth in a church, indicating the secrecy of the confessional.