Knowing. Acquainted with another’s meaning or proceeding. The rattling cove is fly; the coachman knows what we are about.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
To Flux * Fly-1Nathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes