Knowing. Understanding another’s meaning. The swell was flash, so I could not draw his fogle. The gentleman saw what I was about, and therefore I could not pick his pocket of his silk handkerchief. To patter flash, to speak the slang language. See Patter.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1736 dictionary of canting and thieving slang.
To Flare * Flash PanneysNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes