Economist had a good article recently about how this came to be 15 years ago: https://www.economist.com/interactive/christmas-specials/202...

Long story short he got lucky in the 90s with an inheritance and a publisher and can now devote his life to researching and publishing English slang. It's also interesting to me because it's a project that started as a book but has now migrated successfully to the Internet, both for publishing the dictionary and for doing research for updates to the dictionary.