So I've been thinking about it since last week after Pandora re-named Guese to Petelguese when he absorbed the Witch Factors. Based on that and the fact that when Satella joined Echidna's TP, Typhon called her Tella (or Terra).
----START OF THEORY----
1) Terra in Roman myths is Mother Earth. So let's say Satella's true name was Terra and after she absorbed the Witch Factors, Pandora named her Satella (Saterra).
Sa as an Egyptian hieroglyph means "Protection". As we know, she was incompatible with those factors and nearly covered the world with her shadows. So maybe that "Sa" implies that the WoE tried to "Protect" the world (earth) by covering it with her shadows.
2) Sa is also a chinese surname coming out of Semu. According to the Wikipedia, as a name, it meant "Assorted Categories" emphasizing the Ethnic Diversity between Semu people. And we know Satella is a half-human half-elf so the "Sa" gives emphasis on that fact.
----END OF THEORY----
About the 2) tbh in the beginning i thought of it as a way to like seperate Elves from everyone else since everyone hated Elves after what Satella did and they were forced to hide themselves. But it didn't really make sense with the word's meaning to me since it doesn't tend on racism. But correct me if i'm wrong.
Hope i didn't kill many of your brain cells (since i'm anime-only with a little extra knowledge from the Wiki) and i can't wait to see other people's opinion!! Have a good day!