sub rosa

Sub rosa comes from the Latin, literally "under the rose," from the ancient association of the rose with confidentiality, the origin of which traces to a famous story in which Cupid gave Harpocrates, the god of silence, a rose to bribe him not to betray the confidence of Venus. Hence the ceilings of Roman banquet-rooms were decorated with roses to remind guests that what was spoken sub vino (under the influence of wine) was also sub rosa.