Conceptual Development: The earliest mention of this character named him ᴹQ. Árundil (SD/401). In Lord of the Rings drafts from the 1940s, this name first appeared as (Noldorin?) Ithildor >> Isildor (RS/271), soon revised to ᴹQ. Isildur (RS/320). The name Isildur also appeared in a marginal note in The Etymologies from the 1930s (Ety/NDŪ).
isildurmasculine name. *Servant of the Moon
Isil (þ) place-name "Moon" (FS; SA:sil, Appendix E, SD:302, SIL; also defined as "the Sheen" under THIL); Isildur masc. name., *"Moon-servant" (SA:sil, Appendix A, NDŪ)