Where does “ещё” come from?

ещё (Russian) comes from Proto-Slavic *ešče, from Proto-Indo-European *h₁esk(ʷ)e(h₁).

ещё (Russian): yet, more; else; still

Definitions

Step	Language	Word	Meaning
1	Proto-Slavic	*ešče	—
2	Proto-Indo-European	*h₁esk(ʷ)e(h₁)	—

ещё — Etymology, Origin & Word History | Etymology Explorer