Sub rosa (Neo-Latin for "under the rose") is a Latin phrase which denotes secrecy or confidentiality.
The rose has an ancient history as a symbol of secrecy.
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] This was inherited in later Christian symbolism, where roses were carved on confessionals to signify that the conversations would remain secret.
[1] The phrase entered the German language (unter der Rose) and, later, the English language, both as a Latin loan phrase (at least as early as 1654) and in its English translation.