I believe 'Darth' means Dark.
Sith change their names on becoming Sith to honour Darth Bane. He created the 'always two there are' practise, which was the only way they could survive. Before that, when there were many Sith, they were not all called Darth something. One of them, for example, was Exar Kun (the first to ue a two-bladed lightsaber.)