MEANING:
adverb: Secretly, privately, or confidentially.
ETYMOLOGY:
From Latin sub (under) rosa (rose). Earliest documented use: 1654. The English term "under the rose" is also used to refer to something in secret.
USAGE:
"'Much of this goes on sub rosa and never comes to public view,' said Wesley Wark."
Peter Goodspeed; Vladimir Putin's Support of Spying; National Post (Canada); Jan 23, 2012.