It’s sort of important to use the authentic language when writing historical fiction, or at least to try to be authentic. In reality, who knows what they really said. I’ve assembled a few resources that I use.
There are a couple of books of regency slang available on google books, and a number of sites that clone them. This site is one of my favorites. I have to watch the social class, because they mix thieve’s cant with upper class slang. Lady Dalrymple is too much of a “Gentry Mort” to attend a “Bowsing Ken” for her Ratafia. The company might give her the vapors and she’d need her vinegar. Rather unlikely that she’d even understand the words, much less use them.
http://www.etymonline.com this is one I use to make sure I’m not putting in modern slang. In combination with the google ngram viewer, you can catch most anachronisms.
https://books.google.com/ngrams/graph?content=biscuit+breaks&case_insensitive=on&year_start=1800&year_end=2008&corpus=15&smoothing=7&share=&direct_url=t1%3B%2Cbiscuit%20breaks%3B%2Cc0 This goes directly to the literature for usage counts. It can be surprising. I’ve occasionally found invented slang, such as the phrases Georgette Heyer put into her books to trap plagiarists and paraphrasers.