Where does “осъпа” come from?

осъпа (Old East Slavic) comes from Proto-Slavic osъpa, from Proto-Slavic -a, from Proto-Indo-European -ōd.

Ancestry of “осъпа”, step by step

StepLanguageWordMeaning
1Proto-Slavicosъpasmallpox
2Proto-Slavic-aid
3Proto-Indo-European-ōd—
Every word from Proto-Indo-European -ōd →Every word from Proto-Slavic -a →Every word from Proto-Slavic osъpa →