Concise Bible Dictionary:

This seems to be a symbolical name for Nineveh. The word signifies “established”; as in the margin. That which counts itself as established shall be carried away captive (Nah. 2:77And Huzzab shall be led away captive, she shall be brought up, and her maids shall lead her as with the voice of doves, tabering upon their breasts. (Nahum 2:7)).

Jackson’s Dictionary of Scripture Proper Names:

he was established