Under the rose (sub rosa). In strict confidence. Cupid gave Harpocrates (the god of silence) a rose, to bribe him not to betray the amours of Venus. Hence the flower became the emblem of silence. It was for this reason sculptured on the ceilings of banquet-rooms, to remind the guests that what was spoken sub vino was not to be uttered sub divo. In 1526 it was placed over confessionals. The banquet-room ceiling at Haddon Hall is decorated with roses. (French, parler sous la rose.)

credit to THE DICTIONARY OF PHRASE AND FABLE BY E. COBHAM BREWER