A cryptolect, an argot, and/or a set of secretive low-
key code word terminologies (also known as thieves' argot, rogues' cant, or peddler's French) used by thieves, beggars, vagabonds, criminals and hustlers in primarily of Great Britain in the early 16th century onwards until falling completely out of use in all language and literature by the late Victorian Era.
Simply put, it was the speakeasy, lowkey language and/or code word in itself, at the
time used to avoid tipping off less knowledgeable agents of law enforcement.
The term was likely devised by the self declared "King of the Gypsies of Derbyshire's
Devil's Arse", now known Peak Cavern, by either at the
time Richard Skelton and/or/both his soon to be successor Cock Lorel (his real name a mystery, lost to
time, but his pseudonym meaning rascal leader) and their band of misfits who either preyed upon the
rich or all folk of the Shires of Britain at the
time.
The term became extremely
popular after literature of the 16 and mostly 17th centuries would depict the lives, both fictional and real, of these rogues, both recent and historic at the
time, by modernfolk who could read to those who could not, usually at less reputable pubs or "public houses"at the
time.
In modern times, it's used primarily by geeks who play tabletop RPGs or those who have a keen interest in histoical literature. Some secretive clever groups of our society today might adopt the code for their own use in secretive deeds.
Thief A: "oi good fortune, brethren.
Spot a pint for a recount of my most recent misfor-?"
Thief B: "nay, AN' HUSH! ...now see gent yonder
table there in the nook? Dressed
fine, he is, aye? Alone? Surly well endowed in coin an no match us? Thieves' can't."
Thief A: "Behind the pub when he goes to take the piss? We can purloin the loot and disperse, if you can muster up that pint fo'a fellow roué?"