Thieves Cant

Thieves Cant is a linguistic style used by primarily criminal and/or poorer people along the coast. Although some scholars argue that it should be merely considered a dialect rather than a unique language, none can argue that Thieves Cant is near-indecipherable to anyone not in the know.
The language started as a series of codewords used by thieves to discuss their activities in public without alerting their surroundings. Many words in Common became slang, such as "Cake" becoming code for "Stolen Goods", or "Wide-eyed" for "Target/Mark". As these enterprises spread across the coast, so did the language, which started to incorporate words from other languages. Nowadays, Thieves Cant is a combination of Common, Elvish, and Dwarvish, where none of the words they've adopted has the same meaning as their original language.

"Now, gentlemen! GEN-TLE-MEN! I have listened to you prattle on for long enough about this topic, and as I have no desire to listen to it again when we meet again, I shall put this argument to rest.
Although it is true that Thieves Cant is an amalgamation of several languages, it is wholly inappropriate to discredit it in its entirety as a language simply based on that. The common tongue includes several words taken from other languages - such as the "-ware" suffix from Elvish. We know that languages naturally change and blend over time, and this argument would suggest that our ancestors would say that we ourselves don't speak common because of the changes that has occured in it.
As for it's heavy use of coded language, I would argue that ALL languages make use of innuendo and codes. Have you never been schoolboys? No one here has laughed at a double entendre? No? For shame, none of you have lived.
On a personal note, as one of the few here which has truly studied Thieves Cant as a language, let me say that I enjoy speaking it for one simple reason - the drama. Sure, it is often both professional and practical to speak in common to communicate with the gentry, but the Cant is full of phrases and figures of speech which seem crafted to upset the noble-minded amongst us. For its one thing to say 'please go away', and a completely different thing to say 'step off, bitch'"

- 

Porto Amberman, Linguist