Where does “-stvo” come from?

-stvo (Old Czech) comes from Proto-Slavic -ьstvo, from Proto-Balto-Slavic -istwas.

Ancestry of “-stvo”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	-ьstvo	Deadjectival, forming nouns denoting a condition...
2	Proto-Balto-Slavic	-istwas	—