In the Etymologies, the name Dior is said to mean "successor" in Doriathrin. The name derives from Primitive Quendian ndeuro ("follower, successor"), from root NDEW.
Sindarin
dior
masculine name. Dior
Dior
Dior
dŷr
successor
*dŷr (i dhŷr, o ndŷr), no distinct pl. form except with article (i ndŷr). Suggested Sindarin form of Doriathrin dior.
dŷr
noun. successor
Derivations
The name of Thingol’s grandson and successor (S/180).
Conceptual Development: The name G. Dior appeared in the earliest Lost Tales, although without translation (LT2/240). In The Etymologies from the 1930s, Tolkien designated the name Dior as Ilkorin, a derivative of the primitive form ᴹ✶ndeuro with the meaning “successor” (Ety/NDEW), though initially the name was marked Ossiriandic (EtyAC/NDEW).
It is unclear whether this translation remained valid in Tolkien’s later writing, especially since he later added the sobriquet S. Eluchíl “Thingol’s Heir” to Dior’s name (S/188). Furthermore, the original etymology would not be valid for Sindarin, since primitive [[os|[eu] became [iu]]] in Old Sindarin and then became [ȳ] in Sindarin. Tolkien never provided another translation or etymology for this name, however.