An element in several river names such as Q. Anduinë and Q. Nunduinë, the equivalent of S. duin. It did not survive as an independent word in Quenya:

Common Eldarin bases DUY and LUY, for instance, were distinct. DUY meant “to flood, drench, inundate”, but LUY was the base of words for “blue”. Both would become LUY in Quenya. Which probably accounts for the disappearance from Quenya of C.E. *duinē “large river (liable to flood surrounding land)” seen in [S.] Anduin “long river” and Baranduin “brown river”: it became identical with [Q.] luine adj. “blue” (VT48/23).

In fact, its use in Quenya river names may have been a later loan from Sindarin.

Conceptual Development: In one place Tolkien did consider the suffix’s survival as an archaic independent Quenya noun †nuine, but Tolkien rejected the note where it appeared, replacing it with the above (VT48/30 note #2).