A noun for “grandchild, descendant” in The Etymologies of the 1930s derived from the root ᴹ√ÑGYO or ÑGYON of the same meaning (Ety/ÑGYO). The ndy in this word is because velars became dentals before in Ancient Quenya, so that ñgy > ndy. The word {indyo >>} inyo appeared with the gloss “grandson” in a deleted marginal noted by the entry for the root ᴹ√YO(N) “son” (EtyAC/YŌ), and indyo “grandson” appeared in notes on The Feanorian Alphabet from both the 1930s (PE22/23) and 1940s (PE22/52). In these Feanorian Alphabet notes as well as in The Etymologies, the word indyo was the name of the tengwa 2Ô [ndy] (PE22/23, 52; EtyAC/ÑGYŌ).

Conceptual Development: In the Qenya Lexicon of the 1910s, ᴱQ. yondo was glossed “male descendant, usually (great) grandson” (QL/106), but in later writings yondo was used for “son”.

Neo-Quenya: It is possibly Tolkien intended indyo to be used only of male grandchildren, but since we have no word for “granddaughter”, I think it is best to assume it can be used for grandchildren (or indeed any descendant) of either gender for purposes of Neo-Quenya.