The hidden city of the Noldor in Beleriand, translated “Hidden Rock”, an adaptation of its Quenya name Ondolindë “Rock of the Music of Water” (S/125). Tolkien stated that the name Gondolin was properly “neither Sindarin or Noldorin [Quenya]” (PE17/29), but the Sindarized name was reinterpretated as a combination of gond “stone” and dolen “hidden” (WJ/201).
Conceptual Development: The name G. Gondolin appeared in the earliest Lost Tales, but at this stage it was translated “Stone of Song” (LT2/158). This was the same meaning as its early Qenya name Ondolinda, with the second element being G. dólin “song” (GL/29, 41; LT1A/Gondolin). In The Etymologies from the 1930s, Tolkien revised the meaning of N. Gondolin to “heart of hidden rock” (Ety/DUL), setting the stage for the later derivation described above.
hidden rock; gond (“great stone, rock”) + dollen (p.p. of doltha- “conceal”); [Etym. DUL-] - “heart of hidden rock”; gond (“great stone, rock”) + dol (from dollen p.p. of Doltha “conceal”) + ind (“inner thought, heart”); S name for Q Ondolindë (“the Rock of the Music of Water”).