"You don’t know what I’m talking about, do you?"
It was said to be used by vagabonds, Jews, Gypsies, and thieves.
And French butchers, Irish Travelers, Yemeni businessmen, and Parisian prostitutes.
And beggars, carnies, gay men and the Sheikh Mohammadi peddlers of Afghanistan.
It’s been called "peddler’s French" or "cant", a specialized jargon used by those who want to exclude others.
An academic named Daniel Heller-Roazen has written a book about it called Dark Tongues: The Art of Rogues and Riddlers.
As he wrote in an article (Learn to Talk in Beggars’ Cant) in The New York Times:
The truth is that wherever people speak a language, they find ways to modify it according to set rules. A cryptic idiom may be developed for the purposes of a game, to enable a literary activity, to facilitate a new society or to implement a political project. Its secrets may be innocuous or harmful. What is certain is that speech can always be both a basis of understanding and a means of distortion.
There’s a review by Jacob Mikanowski in Slate (The Tongues of Rogues. How secret languages develop in closed societies):
Practically every major European language had a canting speech of its own… At the very moment when Renaissance humanists and authors were trying to standardize and purify their vernaculars into literary languages, they were discovering that they were already corrupt, riddled with dialects whose sole purpose was to deceive, to defraud, and to conceal.
The same is true today.