Early Primitive Elvish
nū
root. *under
nuhu
root. bow, bend down; stoop, sink
ū
root. under
There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
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.