Modern words in Fantasy/Historic novels
You also don’t want to use slangs or “hip” phrases.
E.g: You wouldn’t have someone say “Yolo” in a historical setting, would you? Or “dude” or “I pity the fool” before Mr T’s time?
If you’re unsure of the usage and where it originated from, online dictionaries usually can tell you when the word was used, and how popular it was. Be aware of idioms. While the words itself were invented before your book setting, the way it’s used, may be modern.
E.g: A friend of mine had trouble because she wrote ‘falling in love’ and that phrase was invented after her novel…
You can also use slang online dictionaries, which give you a rough time they were used as well.
If you really can’t find it, then don’t use it. Or if you’re writing a fantasy novel, make your own! Make it so it also builds the world!
If you’re writing a historic fiction: it’s best to look up words for their use in the period you’re using. Words change meaning over time, and you don’t want to be caught out using the modern version. (See: Awesome, Terrific etc)
If you’re writing fantasy fiction: it’s best to establish a rule. Do you have ducks in your world?
You wouldn’t really use “Jesus Christ” as a swear word unless the residents in your world know who Jesus Christ is, and how that swear word came about. Why not use a swear word that defiles a religious person in your world? (My favorite was always “Andraste’s dimpled buttcheeks” in the Dragon Age world)
Or use a word that sounds like our swearword like in Battle Star Galactica, where they use “Frak” in place of… you know which word I’m talking about.