This word, which also appears on the map of Middle-earth in LotR, is glossed as "blue" in The Etymologies, but Tolkien later rejected this meaning (as luin was already used in that sense). He then proposed several explanations for it, including the possible adaptation of a Dwarvish name into Sindarin, but he apparently never reached a definitive solution.
Noldorin
lhû
noun. time, occasion
lhû
noun. a time, occasion
lhûr
noun. slumber
lhûn
place name. Blue River
lhûn
adjective. blue
lhûg
noun. dragon
lhûg
noun. snake, serpent
lhûn
adjective. (unknown meaning)
lhûth
noun. spell, charm
lhûth
noun. spell, charm
lhûtha-
verb. to enchant
rhû
noun. loud-sound, trumpet-sound
amlug
noun. dragon
am-
prefix. snake
amlug
noun. dragon
borth
?. [unglossed]
clei
?. [unglossed]
coen
?. [unglossed]
diragas
?. [unglossed]
elw
adjective. (pale) blue
lhimlug
noun. fish-dragon, sea-serpent
mauth
?. [unglossed]
menwed
?. [unglossed]
nella-
verb. to sound (of bells)
nith
?. [unglossed]
nithrad
?. [unglossed]
níth
?. [unglossed]
porennin
?. [unglossed]
rhomru
noun. sound of horns
telwen
?. [unglossed]
A noun appearing as N. lhûr “slumber” in The Etymologies of the 1930s, cognate to ᴹQ. lóre, derived from the root ᴹ√LOR “sleep” (EtyAC/LOS). Tolkien deleted this form when he revised the root to ᴹ√LOS.
Conceptual Development: G. lûr “slumber” appeared in the Gnomish Lexicon of the 1910s (GL/55), clearly based on the early root ᴱ√LORO “doze, slumber” (QL/56).
Neo-Sindarin: I’d restore this word as ᴺS. lûr “sleep, slumber”, since I retain the connection between the root √LOR and sleep; see the entry on √(O)LOR for further discussion. Hints of this word can be seen in the pure Sindarin name S. Lothlúrien for Lothlórien “Dream-flower” (PE17/48).