Except that the word Sith isn't even spoken in the original trilogy. According to Wiki, it's first appearance was in the novelization of the movie (which was published 76, months before the movie even came out), and was borrowed from the name of large poisonous insects in Edgar Rice Burrows' "John Carter of Mars" series.

Okay, haha, that'll teach me to read through the entire post first. They're all jokes.