Under the rose: privately or secretly. The rose was, it is said, sacred to Harpocrates, the God of silence, and therefore frequently placed in the ceilings of rooms destined for the receiving of guests; implying, that whatever was transacted there, should not be made public.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Ropes * Rosy GillsNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes