Merriam-Webster's Word of the Day for March 12, 2010 is:

sub rosa • \sub-ROH-zuh\  • adverb
: in confidence : secretly

Example sentence:
The private investigator met sub rosa with his client to show her photos of her husband rendezvousing at various local establishments with another woman.

Did you know?
"Sub rosa" literally means "under the rose" in New Latin. Since ancient times, the rose has often been associated with secrecy. In ancient mythology, Cupid gave a rose to Harpocrates, the god of silence, to keep him from telling about the indiscretions of Venus. Ceilings of dining rooms have been decorated with carvings of roses, reportedly to remind guests that what was said at the table should be kept confidential. Roses have also been placed over confessionals as a symbol of the confidentiality of confession. "Sub rosa" entered the English language in the 17th century, and even before then, people were using the English version, "under the rose." Earlier still, "unter der Rose" was apparently used in Germany, where the phrase is thought to have originated.