Slang; dog, or dog's Latin; gibberish.
“What did actually reach his ears was disguised so completely by the
use of cant words and the thieves' Latin, called slang, that he ...
could make no sense of the conversation.” —
“He can vent Greek and Hebrew as fast as I can thieves' Latin.” —
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894