George Lucas claimed that Vader is a corrupted form of the Germanic word Vater, meaning "father", after the release of Empire Strikes Back in 1980. The word "Darth" stems from the English word "dark", so Darth Vader really means "dark father". The clues were there all along. That's if you believe that, of course. It's probably more likely that the name Vader happened to sound like the Germanic word for father, which influenced Lucas's decision to write this into the story.