A simple definition can be used here. Cant is slang, vernacular common to a certain class or region, commonly the underworld. In this case, I'm talking about the speech common to the Regency underworld.
What a great opportunity to plug one of my favorite books, the 1811 Dictionary of the Vulgar Tongue. A dictionary of Buckish Slang, University Wit, and Pickpocket Eloquence. Seriously. It is probably one of the most informative books I've ever read. While I don't use slang very often in my own writing, I have had occasion to use some words. Heck, I've used some of them not even realizing they were slang. :o)
A few random entries from the aforementioned book (imagine quotes around these):
- Addle Pate. An inconsiderate foolish fellow.
- Bread Basket. The stomach; a term used by boxers. I took him a punch in his bread basket; i.e. I gave him a blow in the stomach.
- Brother of the Quill. An Author. *
- Clanker. A great lie.
- Cull. A man, honest or otherwise. A bob cull; a good-natured, quiet fellow. Cant.
- None-Such. One that is unequaled : frequently applied ironically.
- Rum Doxy. A fine wench. Cant.
- Smithfield Bargain. A bargain whereby the purchaser is taken in. This is likewise frequently used to express matches or marriages contracted solely on the score of interest, on one or both sides, where the fair sex are bought and sold like cattle in Smithfield.
*I just had to include this one. heehee :oP