According to recent studies, the Darth title has recently been revealed as originating from the Rakata Infinite Empire. D'arth was a term for their emperor, but also meant triumph over death or immortal.
It was originally thought that the title was a shortened version of Dark Lord of the Sith, but the revelations of how much of the Infinite Empire's culture was passed down to the Sith discounts this theory.
As of 3/14/10, TSL is restored. The Sith Lords Restored Content Modification by Stoney and Zbyl has been finished and can be downloaded here