Nargûn is the Dwarvish name for Mordor, and means roughly "Black Land". It shows the same suffix as Tharkûn, which is Gandalf. Apparently then, -ûn is used to refer to a specific entity characterized by the root meaning. Think of it as saying "The Black One".
In Arabic, the names of towns, cities, and countries are usually feminine in form. Because Tharkûn is Gandalf and therefore male, and Adunaic also uses <û> as a masculine marker, it's a bit perplexing as to why Nargûn would use the same suffix. However, Mordor would be very closely associated with Sauron. The dwarves may have used the same name for both, or even considered them to be almost synonomous. The association with Sauron could also have simply been enough reason for the dwarves to assign it a masculine name, and in this case perhaps they still used a different name for Sauron himself. The use of names that are feminine in form for countries in Arabic is not an absolute, so it's not impossible that the connection of Sauron and Morder had nothing to do with the form of Nargûn
It's interesting here that "black" appears in the form narg-, rather than narag as seen in Narag-zâram above. The extra <a> could possibly be an epenthetic schwa, but the placement according to Hebrew phonotactics (from my understanding) would actually place it as something more like Nargazâram. On the other hand, Adunaic's dual inflection is -at, and Tolkien says that it tends to "show... suppression of the final vowel before the suffix". For example zadan → zadnat. The same sort of thing may be happening here. I would speculate that perhaps the initial syllable in narag is accented, rather than the final one. So, that leaves the final syllable susceptible to change. This is different from what we see in Gabilân. In gabil, the second syllable must be stressed, allowing it to retain the <i> in Gabilân, or there is some other reason that I can't determine. Perhaps this simply shows that stress is different in words that have two of the same vowel in the template, as in CaCaC in narag.
The Return of the Shadow, pg 466
Parma Eldalamberon XVII: Words, Phrases and Passages, pg 37