Though, for many Internet denizens, Urban Dictionary is the go-to source for slang elucidation, Green's Dictionary of Slang is newly available online, and it offers a far more reliable compendium of modern and old-timey slang words and phrases.
Green's Dictionary of Slang isn't new. In fact, it has been the most reliable source of academic research into local linguistic variants for years, with its hard copy dictionary set gracing library shelves in universities' English departments perennially. In an acknowledgment of the modern times that the repository's slang words often connote, Green's Dictionary of Slang is now available online and in full, free for any and all Internet users.
Unlike Urban Dictionary, Green's is not user-submitted, requiring citations and vetting for all entries. Nonetheless, Green's contains over 100,000 words with over 400,000 citations from throughout the history of the English language.