Where does “жучище” come from?

жучище (Russian) comes from Russian -и́ще, from Proto-Slavic -išče, from Proto-Slavic isk-je.

Ancestry of “жучище”, step by step

StepLanguageWordMeaning
1Russian-и́щеАttached to nouns to form augmentatives
2Proto-Slavic-iščeDenominal, denoting places or spaces; Denominal,...
3Proto-Slavicisk-je—
Every word from Proto-Slavic isk-je →Every word from Proto-Slavic -išče →Every word from Russian -и́ще →