The word Sith was already in the early drafts of Star Wars, in the Journal of the Whills. For some reason the actual word never made it into the final scripts of the OT. But it's a genuine pre-ANH SW word. Just read the first drafts of SW.
EU aside this is the history of the sith presented in the Phantom Menace novel:
Darth Sidious stood high on a balcony overlooking Coruscant, his concealing black robes making him appear as if he were a creature produced by the night. He stood facing the city, his eyes directed at its lights, at the faint movement of its air traffic, disinterested in his apprentice, Darth Maul, who waited to one side.
His thoughts were of the Sith and of the history of their order.
The Sith had come into being almost two thousand years ago. They were a cult given over to the dark side of the Force, embracing fully the concept that power denied was power wasted. A rogue Jedi Knight had founded the Sith, a singular dissident in an order of harmonious followers, a rebel who understood from the beginning that the real power of the Force lay not in the light, but in the dark. Failing to gain approval for his beliefs from the Council, he had broken with the order, departing with his knowledge and his skills, swearing in secret that he could bring down those who had dismissed him.
He was alone at first, but others from the Jedi order who believed as he did and who had followed him in his study of the dark side soon came over. Others were recruited, and soon the ranks of the Sith swelled to ~ than fifty in number. Disdaining the concepts of cooperation and consensus, relying on the belief that acquisition of power in any form lends strength and yields control, the Sith began to build their cult in opposition to the Jedi. Theirs was not an order created to serve; theirs was an order created to dominate.
Their war with the Jedi was vengeful and furious and ultimately doomed. The rogue Jedi who had founded the Sith order was its nominal leader, but his ambition excluded any sharing of power. His disciples began to conspire against him and each other almost from the beginning, so that the war they instigated was as much with each other as with the Jedi.
In the end, the Sith destroyed themselves. They destroyed their leader first, then each other. What few survived the initial bloodbath were quickly dispatched by watchful Jedi. In a matter of only weeks, all of them died.
All but one.
Darth Maul shifted impatiently. The younger Sith had not yet learned his Master's patience; that would come with time and training. It was patience that had saved the Sith order in the end. It was patience that would give them their victory now over the Jedi.
The Sith who had survived when all of his fellows had died had understood that. He had adopted patience as a virtue when the others had forsaken it. He had adopted cunning, stealth, and subterfuge as the foundation of his way-old Jedi virtues the others had disdained. He stood aside while the Sith tore at each other like kriks and were destroyed. When the carnage was complete, he went into hiding, biding his time, waiting for his chance.
When it was believed all of the Sith were destroyed, he emerged from his concealment. At first he worked alone, but he was growing old and he was the last of his kind. Eventually, he went out in search of an apprentice. Finding one, he trained him to be a Master in his turn, then to find his own apprentice, and so to carry on their work. But there would only be two at anyone time. There would be no repetition of the mistakes of the old order, no struggle between Siths warring for power within the cult. Their common enemy was the Jedi, not each other. It was for their war with the Jedi they must save themselves. The Sith who reinvented the order called himself Darth Bane. A thousand years had passed since the Sith were believed destroyed, and the time they had waited for had come at last.