Unpopular opinion, undercommon and thieves cant should be the same language
Especially in settings like waterdeep where the Underdark is so intermingled with the criminal element that one of the city's biggest crime bosses just has a well that leads to it in the middle of his public front bar.
The whole point of thieves cant is that it seems like 'normal' speech to outsiders, though. It just hides illicit meanings within, while sounding like very bland/innocent common (or whatever is used as the mask). Undercommon is immediately noticeable as foreign and pointed out.
Unpopular opinion, undercommon and thieves cant should be the same language
Especially in settings like waterdeep where the Underdark is so intermingled with the criminal element that one of the city's biggest crime bosses just has a well that leads to it in the middle of his public front bar.
The whole point of thieves cant is that it seems like 'normal' speech to outsiders, though. It just hides illicit meanings within, while sounding like very bland/innocent common (or whatever is used as the mask). Undercommon is immediately noticeable as foreign and pointed out.