Where does “nilį́” come from?

nilį́ (Navajo) comes from Navajo ní-, from Navajo ł-, from Proto-Athabaskan *-ɬ.

Ancestry of “nilį́”, step by step

Step	Language	Word	Meaning
1	Navajo	ní-	—
2	Navajo	ł-	—
3	Proto-Athabaskan	*-ɬ	—