A verb in the Gnomish Lexicon of the 1910s glossed “wash”, marked with a “✱” indicating it also served as a primitive root ✱ᴱ√FASA (GL/34).
Gnomish
sô-
verb. to wash clean, bathe
fas-
verb. to wash
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 verb in the Gnomish Lexicon of the 1910s appearing as G. {sûtha- >> sû- “wash clean” and sôtha- “bathe (tr. & intr.)” >>} sô- “wash clean, bathe (tr. & intr.)”, all based on the early root ᴱ√soƀ (GL/68). Likely the long ô is the result of the sound change whereby ou became ō as was usual in Gnomish of the 1910s.
Neo-Sindarin: In later Sindarin ancient ou became ū which produces problematic verb forms, so I would instead adapt this verb as ᴺS. sov- “to wash clean, bathe” as a cognate to ᴺQ. sov- “✱to bathe”. See also ᴺS. whal- “to wash”.