An adjective meaning “secret” or “hidden” appearing in a number of names (UT/157, WJ/256). It is most likely derived from the root ᴹ√THUR (Ety/THUR). It’s unclear why the short u did not become y before i as was usual in Sindarin. Perhaps the u was originally long, as suggested by David Salo (GS/291).
Conceptual Development: Both this word and its Ilkorin precursor thúren seems to be elements of the name Garthurian “Hidden Realm”. As such, the earliest form of S. thurin might be Gnomish furion “secret, concealed” (GL/36) from the earliest versions of that name from the 1910s: G. Gar Furion or Gar Thurion “Secret Place”.
In the 1930s, Tolkien reconceived of these names as Ilk. Garthurian, clearly related to Ilk. thúren “guarded, hidden” as both were derived from the root ᴹ√THUR (Ety/THUR). The long ú in this Ilkorin word is another hint that the Sindarin development might have been similar, although it can’t be identical, because the Sindarin word shows no sign of the a-affection appearing in the Ilkorin form.