🔼The name Telassar in the Bible
The name Telassar belongs to an unknown location and occurs in only one context in the Bible. Some scholars have proposed that Telassar is the same as Ellasar, but that's a mere guess.
During the conquest of Assyria, an official named Rabshakeh went ahead of the main army to persuade target peoples to surrender without a costly fight. When he and his contingent reached Jerusalem, he yelled out to the inhabitants to forgo their loyalty to both king Hezekiah and their God YHWH, and reminded them that the gods of other peoples — including the sons of Eden who were in Telassar (2 Kings 19:12, quoted verbatim in Isaiah 37:12) — didn't save them from the hands of the Assyrians.
What Rabshakeh didn't realize was that YHWH was not Israel's tribal deity but the entity who ran the universe, and who subsequently dropped a killer angel in the Assyrian main camp, leaving 185,000 troops dead. King Sennacherib went home to Nineveh and probably took Rabshakeh and his big mouth with him.
🔼Etymology of the name Telassar
Telassar is generally assumed to be one of four Biblical cities that have names derived from the noun תל (tel), which denotes an artificial hill that comes about from covering up the ruins of a destroyed settlement:
The name Telassar is structured somewhat different than the other three tel-names, but that's possibly because Telassar is an Assyrian town, whereas the other three are Jewish settlements in Babylon (possibly even named by Jews and therefore purely Hebrew).
But when we assume that the first part of our name comes from תל (tel), then the second part would neatly coincide with the name Asshur (Assyria):
For a meaning of the name Telassar both NOBSE Study Bible Name List and Jones' Dictionary of Old Testament Proper Names read Hill Of Asshur, but it should be noted that the Hebrew word תל (tel) denotes destruction and that makes this a curious name to exist within the Assyrian empire. It's more probably that this name denotes a destruction by Asshur rather than of Asshur.
BDB Theological Dictionary does not translate our name, suggests that it's the same as Til-asuri known from Assyrian texts and also doesn't list it among the other tel-names under the noun תל (tel).