Where does “још” come from?

још (Serbo-Croatian) comes from Proto-Slavic (j)ešče, from Proto-Indo-European *h₁esk(ʷ)e(h₁).

Ancestry of “још”, step by step

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