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
thas-
verb. to shave
cautha-
verb. to shave
fas-
verb. to wash
Beware, older languages below! The languages below were invented during Tolkien's earlier period and should be used with caution. Remember to never, ever mix words from different languages!
A verb appearing as G. thas- “shave” in the Gnomish Lexicon of the 1910s (GL/72). It seems to have replaced a rejected word G. cautha- “shave” (GL/24).
Neo-Sindarin: I would retain ᴺS. thas- “to shave” for purposes of Neo-Sindarin as a derivative of the Neo-Root ᴺ√THATH “shave”, where the s is the result of dissimilation away from the initial th. Given the rarity of Elvish beards, this word probably originally applied to shaving the surface of wood.