Darth is a made up name just like Jedi or Sith.
And here is wookieepedia has to say on the subject
It is also worth noting that vader is the Dutch word for father (as well as an Old English variant for "father" cf. OED), and it may be tempting to read the character name Darth Vader as Dark Father, a word-play that may well be the root of the Sith title. However, judging by the origin of the other Sith names, Vader may also possibly be a derivative of the word invader. Furthermore, it is also worth noting that in the original scripts for A New Hope, the name Darth Vader was given to a normal Imperial general.