The name first appeared in the first Star Wars movie.
No, I'm not talking about Episode I, which wasa terrible movie to begin a prequel with new knowledge regarding "midichlorians" and introducing two annoying characters Jar Jar and young Anakin! I'm talking about the one that started it all in 1977. The first recording of the name in film was when Darth Vader was described as being The Dark Lord of the Sith. They only used it once. It wasn't until Episode I's release in 1999, that the name became widely known.
Now you're probably wondering, what is the word "sith" doing in Shakespeare plays? The word's usage appears 24 times in 13 of Shakespeare's plays and sonnets, but is it referring to the hinted Dark Lords who are the cause of so many tragedies found in Shakespeare plays? The answer to that is "no", but it would be cool to think about now wouldn't it?
The word "sith" is another word for "since".
"Sith" this Sith isn't the "sith" we're looking for, we shall look elsewhere.