Where does “нѣсмъ” come from?

нѣсмъ (Old Church Slavonic) comes from Old Church Slavonic не, from Proto-Slavic ne, from Proto-Balto-Slavic nē, from Proto-Indo-European né — not; no.

Ancestry of “нѣсмъ”, step by step

нѣсмъ traces back through two separate lines of ancestry.

via Old Church Slavonic не

StepLanguageWordMeaning
1Old Church Slavonicнеno; not
2Proto-Slavicnenot
3Proto-Balto-Slavicnēnot
4Proto-Indo-Europeannénot; no

via Old Church Slavonic ѥсмъ

StepLanguageWordMeaning
1Old Church Slavonicѥсмъ(we) are
2Proto-Slavic*esmъ—
Every word from Proto-Indo-European né →Every word from Proto-Balto-Slavic nē →Every word from Proto-Slavic ne →