Where does “Bylilly” come from?

Bylilly (Navajo) comes from Navajo bá, from Proto-Athabaskan *-aˑ.

Ancestry of “Bylilly”, step by step

Step	Language	Word	Meaning
1	Navajo	bá	for him/her/it/them, for his/her/its/their sake;...
2	Proto-Athabaskan	*-aˑ	—