If you're writing 18th century dialogue, this website lets you search words and phrases to double-check whether they were in use & meant what you intend. It doesn't include every period-accurate use of a word/phrase, but it certainly helped me separate genuine 18th century grammar from the vague tangle of 💬old-fashioned fancy-speak💬 I've internalized from TV and video games.
Other websites that let you do this:
Johnson's Dictionary Online (thanks @yellowbelliedtoad!) – 1755 and 1773
Green's Dictionary of Slang – 1300s to today
A Classical Dictionary of the Vulgar Tongue – 1788
Feraud's Dictionaire critique de la langue française – 1778
A dictionary of the English and Italian languages by Giuseppe Baretti is a bilingual dictionary from 1790!
