Where does “-iște” come from?

-iște (Romanian) comes from Old Church Slavonic -ище, from Proto-Slavic -išče, from Proto-Slavic isk-je.

Ancestry of “-iște”, step by step

Step	Language	Word	Meaning
1	Old Church Slavonic	-ище	—
2	Proto-Slavic	-išče	Denominal, denoting places or spaces; Denominal,...
3	Proto-Slavic	isk-je	—

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