Gnomish
unt
noun. nothing
unath
noun. nothing
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.
A word appearing as G. unt or unath “nothing” in the Gnomish Lexicon of the 1910s with an adjective form G. untha “no, none” (GL/73, 74). It is related to the negative element G. û (GL/73). It may also be related to ᴱQ. munta “nothing” (PE14/48, 81) and based on the early negative root ᴱ√UMU (QL/98).
Neo-Sindarin: Since √UMU continues to appear in Tolkien’s later writings, I retained ᴺS. unt “nothing” for purposes of Neo-Sindarin, but adapt the adjective form as ᴺS. ont “no, none” with a-affection of u to o. For an alternative, see ᴺS. lad “nothing, zero”.