Grose and Egan (1823) define flash as follows: Knowing. Understanding another’s meaning. The swell was flash, so I could not draw his fogle; [meaning] 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 flash language. Incidently, a dictionary entitled A new and comprehensive vocabulary of the flash language published in 1812 was written by a convict called James Hardy Vaux who was transported to Australia three times before disappearing into obscurity. His work is credited as being Australia’s first published dictionary.
OED defines Cant as follows: noun
4 The peculiar language or jargon of a class: a. The secret language or jargon used by gipsies, thieves, professional beggars, etc.; transf. any jargon used for the purpose of secrecy.
Simlarly, they define flash language as follows: Connected with or pertaining to the class of thieves, tramps, and prostitutes.