Etymologically speaking, it seems feasible that "Darth" is derived from the phrase "Dark Lord of the Sith". This derivation would also conveniently explain the use of "Lord" as an honorific address.
English has a long history of shortening cumbersome titles in a similar manner, such as Bosun, Ma'am / M'Lord, or Gunny. On the contrary, it seems infeasible that the prevalent common language a long time ago in a galaxy far, far away was based on English morphology.