A word appearing as G. thim “milk, after separation” in the Gnomish Lexicon of the 1910s, clearly related to the verb G. thin- “skim” (GL/71-72).

Neo-Sindarin: Although there are no other related words in Tolkien’s later writings, there are also no other alternatives for “milk” in the Sindarin language branch, so I would retain ᴺS. thim “milk” for purposes of Neo-Sindarin. I would use it especially for milk separated from cream. It might be derived from an ancient form such as *thimbē from a Neo-Root *ᴺ√THIM.