Recently a friend and I were discussing the term "Sith" being used to describe those in league with the Dark Side of the force

As far as I can remember the word Sith is never mentioned once in the Original Trilogy so when was it first coined and used? The earliest mention I can think of was a sticker or trading card calling Vader "The Dark Lord of the Sith" around the time of the 1997 Special Editions

Would appreciate any help!