The Quenya name of S. Idril, from which her Sindarin name was derived (SA/ril, PM/346). Her name is a combination of ita- “to sparkle” and the root √RIL “brilliance” (SA/ril, PM/363 note #42) with the feminine agental suffix -llë, perhaps meaning something like “✱Sparkling Brilliance”. This name also appeared as Itarildë, using the suffix -ldë instead (PM/348, PE17/112).
Conceptual Development: For a very long time, Idril’s Quenya name was Irildë (LT2/216, Ety/KYELEP, WJ/235), with its initial element derived from √ID (Ety/ID, PE17/112). After the publication of The Lord of the Rings, Tolkien became dissatisfied with this etymology, as the initial ✶Id- would have developed in Sindarin into Idh-. Tolkien explored several possible solutions to the problem: changing Idril to Ídhril or changing her Quenya name to Itarillë (PE17/112). He eventually settled on the second option (PM/346).
When Tolkien first introduced this new Quenya name, its initial element was íta “very, extremely” from the root √IT “repeat, multiply”, so that her name meant “Very Bright” (PE17/112). Tolkien later changed the meaning of this root to √IT “glitter” with the verb form ita- “to sparkle” (PM/363 note #42), and this was the etymology used by Christopher Tolkien in The Silmarillion appendix (SA/ril).
See Q. Irildë for earlier developments of this name.
Itaril (*Itarill-), Itarillë, Itarildë fem. name, Sindarized as Idril(PM:346, 348; SA:ril). (In earlier sources, Tolkien used Irildë as the Quenya form of Idril, and the proper Noldorin/Sindarin form was then Idhril.)