Sub rosa

Sub rosa is a New Latin term for confidentially, in private, or secretive.

In the Middle Ages, councils would suspend a rose from the ceiling of their chamber to signify that was being said should not be told outside the chamber.

The rose signifies confidentiality under Greek mythology, where Aphrodite gave a rose to her son Eros and he gave it to the god of silence, Harpocrates, to conceal Aphrodite's misbehavior.