What does "Mos" mean? And is there an in-universe explanation for why Eisley was renamed?
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According to the Star Wars Wiki, the city was originally founded by survivors of the crashed "Dowager Queen" that can be seen wrecked in the centre of Mos Eisley and now functions as a hotel.
The city was supposedly named by survivors who were unaware that the planet was already inhabited. When they found out that there were other towns nearby (named Mos Espa and Mos Gamos) they renamed it to fit in with the surroundings.
Quite why they chose the prefix "Mos" isn't mentioned.
I think it means "The" or possibly "Saint" or is at least meant to evoke that sort of feeling.
Remember that Lucas was from Modesto, CA. A vast number of cities in California start with "San" and "Los," but the modest cow town on State Route 99 that Lucas hailed from didn't. So I get the impression that he sort of blended "Los" and "Modesto" and got "Mos."
I imagine somewhere in the back-(lack-of)-woods on Tatooine there's probably a town called Mos Desto.