Where does “-ся” come from?

-ся (Ukrainian) comes from Old East Slavic сѧ, from Proto-Slavic sę, from Proto-Balto-Slavic *sen, from Proto-Indo-European swé — self.

Ancestry of “-ся”, step by step

Step	Language	Word	Meaning
1	Old East Slavic	сѧ	—
2	Proto-Slavic	sę	oneself: myself, yourself, himself, herself,...
3	Proto-Balto-Slavic	*sen	—
4	Proto-Indo-European	swé	self

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