In AD&D thieves cant was a language used by rouge classes to comunicate amongst themselves. It involved using sign language, written text in a non coherent manner (unless you knew the dilect), as well as a verbal form of language somewhat like pig latin.
Alright, why don't we bust out some definitions from www.dictionary.com, here's two you either missed or didn't understand, and since it's the first definition on the page, I'll leave the choice up to you.
The special vocabulary peculiar to the members of an underworld group; argot. The special terminology understood among the members of a profession, discipline, or class but obscure to the general population
Neither of these definitions requires a CANT to be spoken, I'm not denying that cant has a definition in speech, but since Theives Cant was originally the hand signals of the thief class in ADnD, it should be obvious how it appears here.
Thieves Cant is supposed to be a language that doesn't involve words....just hand symbols. It's basically sign language. A rogue can use this to get an idea accross to another rogue without making any noise. So...how do you write a message in Thieves Cant?