A specific jargon from the Middle Ages spread among con men, vagabonds, and criminals. It was called the Thieves’ Cant, and even Martin Luther wrote about it. The separate language wasn’t just slang or jargon, because it survived for hundreds of years, and some of their words still survive today.
The truth is that no one is clear on who started it. All we know is that forms of thieves’ cant began popping up by the 13th century, in various languages in Europe, and were spoken by the lower class as a slang “to the end that their cozenings, knaveries and villainies might not so easily be perceived and known,” as 17th-century English author Samuel Rid wrote. Thomas Harman claimed that the slang was invented around the 1530s by someone who was “hanged all save the head.” What we do know for sure is that over time the language evolved—some say from Welsh Romani, although this too is disputed. It’s also called “peddler’s French," which might indicate a French connection, but is probably just the English insulting the French. Because the creators of cant are unknown and many of their words (deliberately) obfuscated, the roots of many words largely remain a mystery.
That’s what makes thieves’ cant a perfect example of a cryptolect: It’s a secretive jargon that was created specifically to exclude or confuse a particular group—in this case, the cops. Polari, a language spoken by gay Britons in the mid-20th century, is another example of a cryptolect, as is Boontling, which is still being spoken today in Boonville, California.
Read more about the Thieves’ Cant, and see a list of the words used at mental_floss.