Look up sub rosa in Wiktionary, the free dictionary.
Sub rosa (Neo-Latin for 'under the rose') denotes secrecy or confidentiality. The rose has an ancient history as a symbol of secrecy.
History
In Hellenistic and later Roman mythology, roses were associated with secrecy because Cupid gave a rose to Harpocrates (the Hellenistic god of silence) so that he would not reveal the secrets of Venus.[1] Banquet rooms were decorated with rose carvings, reportedly as a reminder that discussions in the rooms should be kept in confidence.[1]