Where does “-ище” come from?

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

Ancestry of “-ище”, step by step

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

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