Where does “собою” come from?

собою (Russian) comes from Old East Slavic собоѭ, from Proto-Slavic *sobojǫ — (with) oneself, each other.

Ancestry of “собою”, step by step

StepLanguageWordMeaning
1Old East Slavicсобоѭwith oneself
2Proto-Slavic*sobojǫ(with) oneself, each other
Every word from Proto-Slavic *sobojǫ →Every word from Old East Slavic собоѭ →