Passing English: Victorian Slang

Every age has its slang, and you can even find "dictionaries" of slang, like this dictionary of slang from late 19th-century England: Passing English of the Victorian Era: A Dictionary of Heterodox English, Slang, and Phrase by J. Redding Ware. The book is free online at the Internet Archive. (And you can read more about the author at Wikipedia: James Redding Ware.)

The book also includes lots of Americanisms of the period, sayings that struck the British of the time as odd and needing explanation. 

Here are two Americanisms, for example: "gaze at the melody" (a variation on "face the music"), and "gee-ru" (euphemistic shortening of "Jerusalem"):

There is also boys' school slang, like this entry about "brolly" as slang for umbrella:

Poke around, and you can find all sorts of things. If you want to tell a story with some good 19th-century slang in it, this is a book that can help!