It is very important that the language in your novel reflects the time and place in which the story is set.
For example, my story is set in Italy. My characters would never “ride shotgun”, a term coined in US in the early 1900s referring to riding alongside the driver with a shotgun to gun bandits.
Do your research! A free tool that I found to be very useful is Ngram Viewer.
You can type any word and see when it started appearing in books. For example…one of my characters was going to say “gazillion” (I write YA) in 1994. Was “gazillion” used back then?
And the answer is…YES! It started trending in 1988 and was quite popular in 1994.
This is really important, especially because language can change in very unexpected ways.
For example, did you know that before 1986 people never said “I need to”?Instead, they were far more likely to say “I ought to”, “I have to”, “I must”, or “I should”.
Don’t believe me?
Anyway, most people won’t notice subtle changes like that. But your reader will notice and be confused when characters in your medieval world use metaphors involving railroads and rockets.
One of the things you can do besides use Google Ngrams is to read books or watch movies written in the time period you want to set your story. The key here is that they can’t just be set in that time period, they have to have been made in that time period.
Also, there’s a Lexicon Valley episode on this very topic which I highly recommend. It’s called Capturing the Past.
SEE ALSO Etymonline. Word origins and when they’re first recorded. So, say I wanted to find out when a “coffee break” became a thing – around the 1950s, as seen in magazine adverts – or characters might talk about more genrallly “taking a break” from the 1860s…