Utumno (stem *Utumnu*-), place-name, the first great stronghold of Melkor in the North (SA:tum, TUB). The etymology apparently has something to do with "very deep" or "very hidden"; the phrase "Utumno the Deep-hidden" in MR:67 may include a rough translation of the name. This later source derives the name from a root meaning "cover over, hide", whereas in Etym it was derived from the root TUB having to do with low-lying things. Whether the primitive form is ¤Utubnu as in Etym (TUB) or ¤Utupnu as in MR:69, the stem-form would be Utumnu**-.
Quenya
utumno
place name. Deep-hidden
Utumno
very deep
The first underground stronghold of Morgoth (S/36), translated as “Deep-hidden” and derived from the ancient form ✶Utupnu (MR/69). As pointed out by Helge Fauskanger (QQ/Utumno), this primitive form implies that its stem form is ✱Utumnu-.
Conceptual Development: In the earliest Lost Tales, this name appeared as ᴱQ. Utumna (LT1/69), probably an elaboration of ᴱQ. tumna “dark or hidden” from the root ᴱ√TUM(B)U (LTA1/Utumna, QL/95). In Silmarillion drafts from the 1930s, it changed to ᴹQ. Utumno (LR/238), and this name also appeared in The Etymologies as a derivative of the root ᴹ√TUB from the ancient form ᴹ✶Utubnu (Ety/TUB). In Silmarillion revisions from the 1950s-60s, Tolkien revised its derivation to the one given above (MR/69).
In The Silmarillion appendix (SA/tum), Christopher Tolkien connected this name to tumbo “deep valley”, but this is more consistent with the 1930s derivation from The Etymologies (ᴹ✶Utubnu < ᴹ√TUB “✱deep”) than the later derivation from the 1950s (✶Utupnu < √TUP “cover over”). @@@ 1950s development reflects older phonetic-rule pn > bn rathern an pn > pt.