Where does “-ę” come from?

-ę (Old Polish) comes from Proto-Slavic -ę, from Proto-Indo-European -ę.

Ancestry of “-ę”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	-ę	Forms nouns for young animals and other...
2	Proto-Indo-European	-ę	—

-ę — Etymology, Origin & Word History | Etymology Explorer