Via Bruce Schneier, a pointer to an article on secret languages:
Around the year 1566, a Kentish gentleman named Thomas Harman became a literary sensation. His Caveat or Warening for Common Cursetors Vulgarely Called Vagabones offered the reader a chance to look into the world of the beggars who walked from town to town seeking alms or shelter and irritating respectable folk. Detailing the dupes and con tricks perpetrated by these vagrants, Harman pointed to a feature that set this community apart: a shared, secret language, known as Pedlars’ French, ‘an unknowne tongue to all but to these bolde beastly bawdy beggers and vaine Vagabonds, beeing halfe mingled with English when it is familiarly talked’. The thieves’ language became famous in the literature of the sixteenth and seventeenth centuries: more simply, it was known as cant.
An interesting article, though I take exception with calling these "secret languages". These are not different languages - they're people using existing language in conjunction with domain-specific meanings to mask their communications. But I suppose "secret language" sounds more exotic and exciting than "jargon".