Where does “бѫ” come from?

бѫ (Old Church Slavonic) comes from Proto-Slavic bo, from Proto-Balto-Slavic ba, from Proto-Indo-European bʰo — emphatic particle: surely, indeed.

Ancestry of “бѫ”, step by step

StepLanguageWordMeaning
1Proto-Slavicbofor
2Proto-Balto-Slavicbafor
3Proto-Indo-Europeanbʰoemphatic particle: surely, indeed
Every word from Proto-Indo-European bʰo →Every word from Proto-Balto-Slavic ba →Every word from Proto-Slavic bo →