_topon. _name of a stream, originally of an Elven-maid loved by Amroth, King of Lórien. Nim is T. nimbi white. -rodel could be S. and mean 'lofty star', but it could also contain an element seen in S. -roth, rod < ROT 'cave', or be feminine, from rodel 'lady, high lady'. Tolkien notes also this name is "suitable to S. sounds and patterns" but is "not clearly etymologizable as S." (PE17:51).
name of a stream
feminine name. Lady of the White Cave
An Elf-maiden, the beloved of Amroth, as well as the name of the stream by which she dwelled (LotR/339). According to Tolkien, this name was of Silvan (Nandorin) origin adapted to Sindarin (LotR/1127). It is translated “Lady of the White Cave” (RC/302) and is a combination of nim “white”, the suffixal form -rod of grod “cave” and the feminine suffix -iel (SA/groth, PE17/49).
Conceptual Development: In Lord of the Rings drafts from the 1940s, this river was first named N. Linglor and the maiden N. Linglorel (TI/222-3), both revised to N. Nimladel >> Nimlorel >> Nimlothel (TI/223, 238-9 note #17, the last of these used only for the maiden). Later still both were changed to their final form Nimrodel (TI/223, 239). In later notes from the 1950s, Tolkien considered interpreting the final element as either “lofty star” or “high lady” (S. rodel) from raud “lofty, noble” (PE17/49).
Word Gloss nim “white” groth “large excavation, delving, underground dwelling, large excavation, delving, underground dwelling; [N.] cave, tunnel, [G.] grot” -iel “daughter; feminine suffix”
Nimrodel means "Lady of the White Grotto".