Aware of a thing. Knowing it. There is no down. A cant phrase used by house-breakers to signify that the persons belonging to any house are not on their guard, or that they are fast asleep, and have not heard any noise to alarm them.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Down Hills * To DowseNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes