How did under the rose come to mean "in secret"?: Information Roundup
Under the rose is a literal translation of Latin sub rosa. Its use in the sense of privately, confidentially or secretly is of obscure origin, but it is believed to be derived indirectly from Greek mythology. In ancient Egypt the sun-god Horus was often represented in statues and pictures as a naked baby sucking his thumb, symbolizing birth or childhood. The Greeks and Romans borrowed this Egyptian god and identified him with their Harpocrates; but they mistook the characteristic thumb-sucking attitude for a finger on the lips and supposed it to symbolize silence. Thus Horus, the god of childhood, was transformed into Harpocrates, the god of silence and reticence, and as such he was a favorite deity among the later schools of mystic philosophy. Cupid, it is said, gave Harpocrates an open rose to bribe him not to disclose the amorous pranks of Venus and thereafter this flower was the emblem of the god of silence and the symbol of secrecy. A rose was sometimes worn in the headgear of an Athenian as a sign of membership in a secret society, One writer says the exact form of the phrase [page 65] sub rosa, "under the rose," was due to the fact that a Greek general once plotted the betrayal of his people to the Persians in a chamber that had this symbol of secrecy sculptured on the ceiling. That may not be authentic, but during the Middle Ages this sign in a council chamber indicated that the sessions were "executive" and the proceeds were not to be disclosed. Sometimes the emblem was placed on the ceiling of a banquet room, or a vase of actual roses was placed in the center of the table, to remind the guests that what was said and done during the entertainment was "off the record" and would not be divulged, The emblem was placed over confessionals as early as 1526 to assure the person confessing that everything was in the strictest confidence. There is reason to suppose, says the Oxford dictionary, that sub rosa and under the rose reached England by way of Germany. No occurrence of the phrase has been found in English earlier than 1546. After the revolution of 1688 the Jacobites adopted the rose as their emblem because their meetings were in secret and their aid to the Stuart cause was sub rosa.