From the introduction:
This dictionary is based on the "Dictionary of the Vulgar Tongue. A Dictionary of Buckish Slang, University Wit and Pickpocket Eloquence." printed for C. Chappel of Pall-Mall, London in 1811, and based on the dictionary compiled by Captain Grose in 1785. Despite its date of publication, it can be used in AD&D campaigns by those who desire a detailed use of thieves cant.
Although the guide specifically is designed for AD&D, in practice there is no game-specific content which would limit the use of the product in any setting where an 1800s thieves cant is needed.