The slang of the Romany folk. Even Bracton uses the word Frenchman as a synonym of foreigner, and it is not long since that everyone who could not speak English was called a Frenchman. The Jews, with a similar width, used the word Greek.
“Instead of Pedlars' French, gives him plain language.” —Beaumont and Fletcher: Faithful Friends, i. 2.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894