In Silmarillion drafts from the 1930s, this was the name of the Elves who remained in Beleriand, so called because they never came to Kôr (SM/85, LR/171). It was a combination of the negative suffix il- with the name of the city Kôr (Ety/AR², LA).
Conceptual Development: In the earliest Lost Tales, the term Ilkorin was used for the whole of Elven-kind who did not travel to Valinor (LT1/196), equivalent to the later terms ᴹQ. Lembi and Q. Avari, though in the Qenya Lexicon of the 1910s, Tolkien at first used the term Teleatka or Teleakta as the name of their language (QL/32, 39). By the time of the Silmarillion drafts from the 1930s, this term referred only to the folk of Thingol in Beleriand (LR/171, 181). In Tolkien’s later writing, the Elves of Beleriand became the Sindarin.