under the rose
Privately, confidentially, or in secret. From the Latin phrase sub rosa, itself referring to the ancient use of the rose as a symbol of secrecy. No, I had no idea they were planning to reject the proposal. They must have reached that decision under the rose. As president, keeping one's tax information under the rose is not illegal, but it is widely seen as ethically questionable.
under the rose
in confidence; under pledge of secrecy. archaic The origin of the rose as an emblem of secrecy is uncertain; the concept may have originated in Germany and there was a similar expression in early modern Dutch. Under the rosse appears in a 1546 State Paper of Henry VIII , with a gloss that suggests that it was then a new or unfamiliar expression. The Latin equivalent sub rosa has also been very commonly used in English since the mid 17th century in this metaphorical sense.