Sub Rosa
Definition: in secret
It’s literally Latin for “under the rose.”
I found a couple reasons why this implies secrecy (from American Heritage):
From the practice of hanging a rose over a meeting as a symbol of confidentiality
And where did this come from? According to Wikipedia:
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. Banquet rooms were decorated with rose carvings, reportedly as a reminder that discussions in the rooms should be kept in confidence. This was inherited in later Christian symbolism, where roses were carved on confessionals to signify that the conversations would remain secret.
Why I Looked It Up
In The Constitution of Knowledge:
His voice, and the sub rosa voices of Princeton faculty who endorsed a letter they disagreed with, were the voices of demoralization.
Postscript
Added on
From On Paper:
Anticipating the likelihood of Yankee chicanery (agreement concealing the actual figures could easily be finagled sub rosa)…