The root ᴱ√Ū⁽¹⁾ “under” appeared in the Qenya Lexicon of the 1910s as a reduced form of {ᴱ√UGU >>} ᴱ√UƷU, with derivatives like ᴱQ. ū “beneath” and ᴱQ. umbe “dale, dell” (QL/96-97). It also had derivatives in the contemporaneous Gnomish Lexicon such as G. um⁽⁾ “lowlying” and G. umbel “a dell” (GL/74). Tolkien noted that ᴱ√Ū also meant “not”, and contrasted ᴱ√Ū¹ “under” with ᴱ√NŪ (ᴱ√NUHU) of similar meaning (QL/68, 96). Indeed, in Tolkien’s later writings, √NŪ/UNU was the usual basis for “under” words, and ᴱ√Ū¹ “under” seems to have been abandoned.
Early Primitive Elvish
mara
root. *sand, soil
gu̯ara-
verb. to dwell
mbara
root. dwell, live
sōđā
noun. house
ū
root. under
ẇaða
root. dwell
A root appearing in the Qenya Lexicon of the 1910s as root {ᴱ√MAŘA² >>} ᴱ√MARA that Tolkien indicated was a dialectical variation or byform of ᴱ√MṚŘṚ [MṚÐṚ] “grind”, with a single derivative: ᴱQ. marilla “pearl” (QL/59). In the contemporaneous Gnomish Lexicon, G. brithla “pearl” was most likely a derivative of ᴱ√MṚÐṚ (GL/24). However, in the m-section of GL Tolkien said “mar and môr probably conceal two roots and a confusion [with] mbara”, with various apparent derivatives of mar- such as G. mar “Earth, ground, soil”, G. marol “sand” and G. Môr “the Earth” (GL/56). Tolkien also had G. mara- “dwell”, but it appeared before a variant (m)bara and so likely was derived from ᴱ√MBARA “dwell, live” as mentioned in the Qenya Lexicon (QL/60). As for ᴱ√MARA² itself, it may have meant “✱sand, soil”.