From BiblePortal Wikipedia

Smith's Bible Dictionary [1]

Huz'zab. (Fixed). According to the general opinion of the Jews, was the queen of Nineveh, at the time when Nahum delivered his prophecy.  Nahum 2:7. (B.C. About 700). The moderns follow the rendering in the margin of our English Bible - "that which was established." Still it is not improbable that, after all, Huzzab may really be a proper name. It may mean "The Zab Country", or The Fertile Tract East Of The Tigris, Watered By The Upper And Lower Zab Rivers.

Fausset's Bible Dictionary [2]

Commonly represented as queen of Nineveh. Rather the Zab country, E. of the Tigris, watered by the upper and lower rivers, Zab Ala and Zab Asfal. A-diab-ene, the best part of Assyria representing the who1e. The "Zab" is named in the inscription of Tiglath Pileser I in the 12th century B.C. ( Nahum 2:7). Gesenius connects it with  Nahum 2:6, "the palace shall be dissolved, and shall flow away" (Henderson) "... though firmly established" (see margin).

Hastings' Dictionary of the Bible [3]

HUZZAB . A word occurring in   Nahum 2:7 . Gesenius derived it from a verb tsâbhabh , and read ‘the palace is dissolved and made to flow down .’ Many recent authorities regard it as from nâtsabh , and tr. [Note: translate or translation.] ‘ it is decreed .’ But Wellhausen and others have considered it a proper name referring to the Assyrian queen, or to the city of Nineveh personified.

W. M. Nesbit.

Morrish Bible Dictionary [4]

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.  Nahum 2:7 .

Holman Bible Dictionary [5]

 Nahum 2:7

International Standard Bible Encyclopedia [6]

huz´ab ( הצב , huccābh , only in   Nahum 2:7 the King James Version and the Revised Version margin): Its meaning is doubtful. According to Gesenius, it is a verb, Hoph. of צבב , cābhabh , "flow," hence, to be rendered with preceding verse, "The palace is dissolved and made to flow down." Wordsworth made it Pual of נצב , nācabh , "fix": "The palace is dissolved, though established." Septuagint renders with the next word, hē hupóstasis apokalúphthē , "The foundation (or treasure) is uncovered." the King James Version, the Revised Version margin and the American Standard Revised Version text make it Hoph. of nācabh , "fix," hence, "It is decreed." Perhaps more probably, with the King James Version and the Revised Version (British and American) text and the American Revised Version margin, it is a name, or noun with the article (or the corruption of such a word), referring either to the Assyrian queen, or personifying Nineveh. No such queen is now known, but Assyriology may throw light. The "name" interpretation accords best with the general trend of the passage, which describes the discomfiture of a royal personage. BDB calls it "perhaps textual error." The Massoretic vocalization may be at fault.