Latin - under the rose: confidentially; secretly; privately.
The tradition of suspending a rose from the ceiling (or painting roses on the ceilings) was to remind those present that anything said there should remain sub rosa, from Medieval council chamber meetings to wine-soaked Ancient Roman banquets.
From the series Seeds of Love and Lust, which re-imagines botanical idioms into a gardening guide for tending to one’s love life. Each illustration corresponds its own idiom, and is accompanied by a piece of advice for the lover/gardener.