1. In secret; privately; confidentially.

1654, from Latin, lit. "under the rose," which was regarded as a symbol of secrecy.
The origin 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.