Oh boy, crime!

• Ruffians 
• Bandits 
• Blackguards

Man, it's weird that criminals have gotten such short shrift in D&D; with how urban most campaigns are, criminals really should have the tools to feature more prominently. We've got some urban criminals here (ruffians) with an included Thieves' Cant glossary, some rural criminals (bandits), and some noble criminals (blackguards).

On the subject of rural criminals, in a lot of languages of Northern and Western Europe "criminal" and "servant/poor/rural" were pretty much synonymous. "Villager" and "villain", are both derived from the same root Middle English root "villanus", for instance; "Churl" was used for both "asshole" and "peasant"; "Boor" could equally mean "ill-mannered" or "herdsman". Under England's "wolfesheed" law, being present in the woods near a road was considered sufficient proof of criminal intent for summary execution.

Thieves' Cant Rant: Make your players explain how they learned it if they want to take it as an exotic language. Thieves' cant was a real thing that was learned through direct, one-on-one apprenticeship, in an era where every part of life was dictated by close personal relationships. Getting in good with a thieves' guild should be a matter of getting an introduction from someone that is willing to stake their reputation (and maybe life) on you being cool; maybe your shady cousin or someone you've done non-criminal work for.

Teaching a non-rogue to cant was functionally punished with death or exile. If you walked up to a crime-y sort of person that didn't know you and started canting at them, they would ask who taught you/who locally can vouch for you, drag you there to check your story, and probably kill you if they didn't like your answer. With most any form of organized crime throughout history, normal people are relatively safe from random murder, but identifying yourself as an unknown and untrustworthy criminal is the absolute best way to get dumped in a river.