Where does “-अ” come from?

-अ (Sanskrit) comes from Sanskrit भाग्य, from Sanskrit भाग, from Proto-Indo-Iranian bʰāgás, from Proto-Indo-European bʰeh₂gós.

Ancestry of “-अ”, step by step

Step	Language	Word	Meaning
1	Sanskrit	भाग्य	luck, destiny, fate, fortune, chance, welfare,...
2	Sanskrit	भाग	part, portion, share, allotment; compartment,...
3	Proto-Indo-Iranian	bʰāgás	division; allotment
4	Proto-Indo-European	bʰeh₂gós	—