Where does “пополниться” come from?

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

пополниться (Russian): to increase, to be replenished, to widen;...

Definitions

to increase, to be replenished, to widen;...

Ancestry of “пополниться”, step by step

Step	Language	Word	Meaning
1	Russian	-ся	Reflexive suffix appended to finite verbs and...
2	Old East Slavic	сѧ	—
3	Proto-Slavic	sę	oneself: myself, yourself, himself, herself,...
4	Proto-Balto-Slavic	*sen	oneself
5	Proto-Indo-European	swé	self

Every word from Proto-Indo-European swé →Every word from Russian -ся →Every word from Proto-Balto-Slavic *sen →