A root appearing in a set of documents referred to as Definitive Linguistic Notes (DLN) written around 1959, with the gloss “hard, difficult” (PE17/154). An apparent draft of this note had a longer gloss “hard, stiff, difficult, cumbrous, slow” for √GUR (PE17/172). In the more extensive notes, Tolkien gave a variety of Quenya (ur-) and Sindarin (gor-) derivatives of this root, but noted that “Sindarin owing to approach of √GUR- to other stems (as ÑGUR- “death”, ÑGOR- “terror, dread”) tends to use gor- in a very strong sense of things very painful and horrible to do; and uses dír- (tough) for lesser efforts” (PE17/154). As an example, Tolkien contrasted S. gorbedui “only to be said with horror or grief, lamentable to tell” versus S. dirbedui “hard to utter, difficult to pronounce”.
Thus it seems as a prefix, this root took on a distinctly negative tone in Sindarin, though some of its other derivatives were more neutral, as in S. gordh “difficult, laborious” and S. gorn “hard, stiff, thrawn”. The last of these was given as a word used for the Dwarves (PE17/46), although there were several other competing forms such as S. dern and S. dorn of similar meaning, all coined around 1959-60.
Counterpart to the root √N(D)IL, this root first appeared in a marginal note from The Etymologies of the 1930s as ᴹ√NDUR or ᴹ√DUR “bow down, bend (down), obey, serve”, an elaboration of ᴹ√NDŪ “go down, sink, set (of Sun, etc.)” (Ety/NDŪ; EtyAC/NDŪ). In the Quenya Verbal System (QVS) from 1948, ᴹ√NDUR was glossed “grow, be dark”, no doubt in connection to N./S. dûr “dark” (PE22/103), but later this “dark” word was connected more directly to √NDU “under, down”. Indeed, in a 1955 letter to David Masson, Tolkien gave √(N)DUR an origin distinct from √N(D)U, basing it on an independent root √DUR “to show special interest in things” (PE17/152); there was also an unglossed and deleted root ᴹ√DUR in The Etymologies of the 1930s (EtyAC/DUR). However, in a 1967 letter to Mr. Rang, -(n)dur again meant “to serve” (Let/386).
Neo-Eldarin: For purposes of Neo-Eldarin, I think it is best to keep the connection between √NDUR “bow down = serve” and √NDU “descend”. As for the semantic distinctions between -(n)dil and -(n)dur, see the entry on the root √N(D)IL for details.