Besides the usual thieves' cant.
Hole Pidgin
Dwarf strongholds and gnome warrens can both expand outward very rapidly. When one inevitably bursts through the walls of another by mistake, they will make haste in beginning commerce between the two settlements. Hole Pidgin is how they communicate with each other; a mishmash of Dwarven and Gnomish vocabulary with very inconsistent grammar and no set word order. It can be difficult to understand even to people who can speak both languages fluently - being immersed in one of these "collision points" for an extended period of time is the only sure way to properly learn it.
Low Magic
This is a cryptolect that can originate from some sort of society ruled by wizards, where those without magic have the least power. Low Magic is a way for those at the bottom of this society to communicate in safety from their cruel wizard overlords. Formed by replacing words from the lexicon of the dominant language with unrelated magic words, anyone with the ability to cast spells even attempting to comprehend it will produce unpredictable and possibly dangerous magic effects. (Use your favorite wild magic table, or roll up a random spell and cast it on a random target.) To those without magic, it's perfectly safe to speak.
The Secret Shanties
These shanties have sections between the chorus and verses for a solo singer to improvise within a strict set of rules. The particular notes and cadences they choose can convey a great deal of information to the members of the crew who are in on the ship's less "above the board" business, while leaving those who don't know what to listen for able to just go about their normal jobs and claim ignorance. There are three popular songs, each with their own rules and code, but any good smuggler or pirate captain will have also cooked up their own bespoke one. Learning to interpret Secret Shanties is about the same difficulty as any other code or secret language; learning to speak it yourself also requires a great deal of musical improvisational skill on top of that.