Where does “б” come from?

б (Translingual) comes from Belarusian Белару́сь, from Old East Slavic Бѣла Русь — {{w|White Ruthenia}} or White Rus.

Ancestry of “б”, step by step

StepLanguageWordMeaning
1BelarusianБелару́сь
2Old East SlavicБѣла Русь{{w|White Ruthenia}} or White Rus
Every word from Old East Slavic Бѣла РусьEvery word from Belarusian Белару́сь