Carn dûm

Found at the most northwestern tip of the Misty Mountains, Carn dûm was the chief stronghold for the Witch-king of Angmar. Its name is something of a mystery. The dûm element is often pointed to as being Khuzdul, parallel to Khazad-dûm. Originally, this was the name of the Dimrill Dale (Kh. Duban Azanulbizar), along with a number of spelling variants such as Carndoom. Even so, it's unclear if this was supposed to be a Khuzdul word or not. 

Regardless, we do know that the original meaning of the name, which we are told in The Return of the Shadow, pg. 433, note 13:


"The name of the vale was first Carndoom the Red Valley…"

This starts to make sense if we look for an Elvish etymology.  Following that line, carn would most likely mean "red", from Quenya carnë and/or Sindarin caran. For dûm we can also find an explanation in an Elvish origin. From The Lost Road, pg 374 we find the example Tumladen, which means “the Level Vale”. Tum means "deep valley, under or among hills”. In response to a question on this possibility Patrick Wynne commented:

The form Carndoom ("Red Valley") and its flock of variants may be found in Return of the Shadow (p. 433, note 13). Doom ("valley") is probably just a spelling variant for dûm in the usual orthography—Cf. dúm ("vale") in Narodûm ("Red Vale") (ibid.), in which the first element is narwnaru (N. "red") (Lost Road: 374). Tolkien apparently intended the form to recall English "doom." The noun without lenition of the initial consonant may be the same as tum "deep valley, under or among hills," seen also in Tumladen ("the Level Vale") (Lost Road: 394), though I cannot account for the difference in vowel length.

Carndoom was originally the Elvish name of the Dimrill-dale. Its subsequent use as the name of the capital of the realm of Angmar makes sense—the first element in Angmar appears to be ang (S. "iron"), so Carn Dûm ("Red Vale") might be an appropriate name for a valley in a land rich in iron deposits. Note that if the second element is mar (Q. "home"), then this is another of those rare mixed forms.

So, we have an Elvish origin for the name and a translation that seems to make sense with what we know about Carn Dûm and Angmar.  Despite not being able to account for the "long u", I feel that it is pretty safe to say that, at the very least, Carn Dûm is probably not Dwarvish in origin.