No, I'm saying Lucas didn't invent the word at all, but borrowed it from an earlier science fiction writer.
The term "Sith" comes from Edgar Rice Burroughs' first "John Carter of Mars" novel. They were described as ferocious "wasp-like creatures."
Everyone talks about Kurosawa and Flash Gordon, but another big inspiration for Lucas is E.E. "Doc" Smith (inventor of the "space opera" genre of sci-fi), writer of the "Lensmen" series, which is where the term "Coruscant" came from (not a planet, but a word meaning "glittering").
The Lensmen were super warriors who had "lenses" that focused their powers (Lucas' abandoned "Kyber crystal" concept used in Splinter of the Mind's Eye), which emanated from the "Cosmic All" (the Force). You could even say Lucas' midichlorians were a (1990's) spin off of this idea.
In fact, Lucas got a lot of his ideas from Smith, including the Jedi and the Force (called different names, but based on similar concepts). After all, Smith wrote before Frank Herbert's "Dune" and inspired his "Prana-Bindu" (Jedi Bendu) training that the Bene Gesserit sisters had, etc.
Nearly all of Lucas' ideas are borrowed from earlier sources. But yes, the Black Knights of the Sith were a group of evil warriors from the early drafts (controlling the evil Bogan "para-force" rather than the Ashla "force of others"), and "Darth" wasn't a title, just a first name for a "grim looking general" who wasn't one of them, wasn't a cyborg, and had no super powers (that was Prince Valorum).
Read "The Secret History of Star Wars" (2008, in print) by Michael Kaminski. It's quite enlightening.