🔼The name Ellasar in the Bible
The name Ellasar belongs to a city or country of unknown location (Genesis 14:1 and 14:9). We know about it thanks to the exploits of king Arioch of Ellasar, who joined kings Amraphel, Chedorlaomer and Tidal on a military campaign to subdue unruly subjects in the south. That lead to the War of Four Against Five Kings, of which we know because Abraham's nephew Lot was caught in the thick of it (Genesis 14:12).
Many locations have been proposed to correspond to Ellasar, from provinces in Babylon to the Hurrian and Hittite territories and even Cyprus or the Telassar mentioned in Isaiah 37:12, but no definite evidence has been forwarded to substantiate any of these claims. We simply don't know.
🔼Etymology of the name Ellasar
Since we don't know where Ellasar was, we also don't know from which language this name came. But the Hebrew scribes had the persistent habit to transliterate foreign names in such a way that they took on a meaning in Hebrew that neatly corresponded to what these scribes thought of them. Thus the name Arioch took on its striking Hebrew appearance, and Ellasar should be treated as a Hebrew name as well.
The first part of our name was possibly meant to correspond to the divine name אל, El, but may just as well reflect any of the other words of that same form:
The second part of our name probably originally reflected the name Asshur (אשור), the main deity of Assyria, but the Hebrew scribes spelled it so that it came to look like it derives from the verb סרר (sarar), meaning to be stubborn or rebellious:
It's unclear what the Hebrew scribes had in mind when they spelled this name this way, but possibly Rebellious God, or rather Onto Rebellion.