A noun in the Gnomish Lexicon of the 1910s glossed “a bath”, related to the verb G. fas- “wash” (GL/34).
Gnomish
soth
noun. bath
fôs
noun. bath
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 noun appearing as G. soth “bath” in the Gnomish Lexicon of the 1910s derived from primitive ᴱ✶sou̯þ- (GL/68), where ou became ō as was usual in Gnomish of the 1910s.
Neo-Sindarin: In later Sindarin, ancient ou became ū which would produce sûth, but since this conflicts with N. sûth “draught”, I would coin a neologism ᴺS. sûn “bath” as the cognate of ᴺQ. súnë “bath”.