Where does “истово” come from?

истово (Russian) comes from Russian и́стовый, from Russian -о́вый, from Old East Slavic -овъ, from Proto-Slavic -ovъ — of, belonging to.

истово (Russian): fervently, ardently

Definitions

fervently, ardently

Ancestry of “истово”, step by step

Step	Language	Word	Meaning
1	Russian	и́стовый	—
2	Russian	-о́вый	—
3	Old East Slavic	-овъ	—
4	Proto-Slavic	-ovъ	of, belonging to

Every word from Proto-Slavic -ovъ →

истово — Etymology, Origin & Word History | Etymology Explorer