A hypothetical root implied by the primitive word ✶lubbu “a clumsy piece or lump” appearing in both the Outline of Phonetic Development (OP1) from the 1930s and the Outline of Phonology (OP2) from the early 1950s serving to illustrate the unvoicing of double stops in Quenya: ✶lubbu > Q. luppo (PE19/45, 92).
The root may be related to earlier ᴱQ. ulumpe “camel”: although its root was given as ᴱ√ULUN(T) in the Qenya Lexicon of the 1910s, Tolkien gave ulumpe- in parenthesis beside the root, indicating it was probably an elaboration on unattested ✱ᴱ√LUPU (QL/97). It might also be connected to G. lub “fat, fat flesh” < ᴱ✶lūpe as well as G. lubi “corpulent” (GL/55), especially given primitive ✶slūbŭ “greasy, fat” from Common Eldarin: Noun Structure of the early 1950s (PE22/82).
Neo-Eldarin: I think it is worth positing a variant root ᴺ√LUP “hump” for Neo-Eldarin in order to salvage Early Qenya camel words.
This root and ones like it were the basis for shadowy things throughout Tolkien’s life, but went through a number of minor conceptual shifts. The earliest appearance of this root was as ᴱ√LUVU in the Qenya Lexicon of the 1910s, with derivatives like ᴱQ. lumbo “dark lowering cloud” and ᴱQ. lūre “dark weather” (QL/57). In the Poetic and Mythological Words of Eldarissa written afterwards, Tolkien gave the root as ᴱ√LUB with a similar set of derivatives (PME/57); phonological developments in both Early Qenya and Gnomish make it very difficult to distinguish ancient voiced stops [b] from voiced spirants [β]. Sign of this root can also be seen in Gnomish words G. lum or glum “cloud”, G. lumbri “foul weather”, and G. luv- “hang, lower, of clouds” (GL/55).
The derivatives of this root in the 1910s seem to connect more specifically to dark weather, but in The Etymologies of the 1930s the root reappeared as ᴹ√LUM with derivatives having to do mainly with shadow, such as ᴹQ. lumbe “gloom, shadow” and N. lhum “shade” (Ety/LUM). These in turn served as the basis for N. Hithlum and ᴹQ. †Hísilumbe >> ᴹQ. Hisilóme interpreted in this period as “Mist-and-Dusk” (LR/406). In earlier writing the second element of ᴱQ. Hisilóme “Misty-gloom” was derived from ᴱ√LOMO (QL/55), whereas in 1964 notes Hithlum was designated “North Sindarin” and given a new etymology as a direct loan from its Quenya equivalent, and thus no longer connected to √LUM (PE17/133).
The last appearance of the root in currently published material was as √LUM or √LUB “shadow, darkness” with derivatives Q. lumbo “dark, shade” and Q. lumbule “shadow” (PE17/168). Q. Luvailin “Shadowmere” (RC/217) is probably related and must derived from √LUB. For purposes of Neo-Eldarin, I would assume that √LUB is the form as the root, as the various lumb- form can likewise be derived from √LUB by way of strengthened ✱lu(m)b-.