I have a hefty tome on my bookshelf - Cassell's Dictionary of Slang, by Jonathon Green. It was published in 1998 and runs to over 1300 pages. However, I was just reading a review in the FT about Green's latest dictionary of slang, and this runs to three volumes and over 6000 pages. Around 110,000 words or phrases are defined.
The FT reviewer includes the phrases 'give someone the mitten' (reject their marriage proposal) and a 'ham sandwich' (meaning 'nothing'). A fogle-hunter was a pickpocket who specialised in stealing silk handkerchiefs. Much of the slang lexicon deals with taboo subjects and in this dictionary there are more words from the fields of drugs, sex and crime than from other areas (although food and money spawn plenty of slang expressions too).
Green provides etymologies where he can, but because so much of slang developed from underground and/or secretive groups or subcultures, sometimes the origins are impossible to trace.
The full review is here and it is an interesting read.
Happy New Year everyone!