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 had to do with covering things for much of Tolkien’s life. The first appearance of the root was unglossed ᴱ√TUPU in the Qenya Lexicon of the 1910s with derivatives like ᴱQ. tumpo “shed, barn” and ᴱQ. tupu- “roof, put lid on, put hat on, cover” (QL/95). Tolkien said this root was “much as TELE”, which in this document was glossed “cover in” (QL/90). The root ᴱ√TUPU also had derivatives in the contemporaneous Gnomish Lexicon such as G. tub- “cover” and G. tump “a shed” (GL/71).
In The Etymologies of the 1930s the root ᴹ√TUP was given as a variant of ᴹ√TOP “cover, roof” with derivatives like ᴹQ. tupse/N. taus “thatch” (Ety/TUP; EtyAC/TOP). In Tolkien’s later writings its most notable derivative was Q. untup- “cover (over)” as in Q. ar hísië untúpa Calaciryo míri oialë “and mist covers the jewels of Calacirya forever” (PE17/73; LotR/377). The root √TUP “cover over” itself was mentioned a couple times in Tolkien’s later writings (PE17/73), though in one place of these places it was given as √TUI “cover over, hide”, but since in that note it was the basis for primitive ✶Utupnu, this √TUI is clearly a malformed ✱√TUP (MR/69).