Dûm
From Tolkien Gateway
dûm means "hall, mansions" and/or "excavations" in Khuzdul.
It is seen in the name Khazad-dûm, the name of Moria
Etymology
Root *D-M
However it is possible that this form is assimilated (from Khazad-. The name Tumunzahar suggests that its original form is tûm.[1]