It has two main meanings:
1. Secretly or confidentially. In ancient Rome, roses were often placed over doorways or tables during secret meetings or discussions. This symbolized that anything said under the rose should be kept confidential.
2. In a clandestine or illicit manner. This meaning is related to the first, as it implies something that is hidden or done in secret.
Therefore, the root word "sub rosa" can be interpreted to mean secret, confidential, or clandestine.