Under common is the lingua franca of the Underdark a vast cave system. A trading pidgin of those that live there and any brave surface soul who trades there.
Theves cant is class ability a coded message telling rogues what nasty criminal things are to do. It's there to make the roleplayers feel like a spy, con artist, or planning a heist.
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.
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