I've collected a lot of great reference books in my research of Diary of Bedlam, but by far my favorite is A Classical Dictionary of the Vulgar Tongue by Captain Francis Grose. First published in 1785, it is a collection of slang words from all corners of society.
Here are a few of the entertaining words and expressions found in this volume:
Bum fodder – toilet paper
To cast up one's accounts – to vomit
Beard splitter – A man given to "wenching"
Dog's soup – rain water
Fart catcher – a valet or footman, from his walking behind his master or mistress
Lazybones – an instrument like a pair of tongs, for old or very fat people, to take something from the ground without stooping
Mantrap – a woman's private parts
Queen Street – a man governed by his wife is said to live in Queen Street
Soul doctor – a parson
Thingumbobs – testicles
Wool gathering - Saying to an absent man, or one in reverie, as in "Your wits are gone a wool gathering."
One thing that's also interesting about the dictionary is to see how many of the words we still use whose meanings are more or less the same as they were over 200 years ago. Expressions like elbow grease, gift of gab, hodge podge, quack, ragamuffin, white lie, and ship shape all hail from this time.