Where does “са” come from?

са (Bulgarian) comes from Old Church Slavonic сѫтъ, from Proto-Slavic sǫtь.

Ancestry of “са”, step by step

StepLanguageWordMeaning
1Old Church Slavonicсѫтъ—
2Proto-Slavicsǫtь—
Every word from Proto-Slavic sǫtь →Every word from Old Church Slavonic сѫтъ →
са — Etymology, Origin & Word History | Etymology Explorer