Melkor's first stronghold. The primitive form is given as Utubnu, derived from a stem TUB (LR:394), not defined as such but yielding a series of words that suggest a basic meaning "deep, lowlying". The prefixing of the stem-vowel is a common feature in strengthened primitive forms; the ending -nu seems to be used nowhere else, but Utubnu is clearly to be understood as "very low [place]". The original cluster bn comes out as m in Nandorin; cf. Quenya Utumno. The development is evidently meant to be _Utubnu > Utumnu > *Utumn > Utum_.
Sindarin
Udûn
utumno
(stronghold of Melkor) Udûn