Where does “*sy” come from?

*sy (Slavic) comes from Proto-Indo-European h₁sónts, from Proto-Indo-European h₁ēs- — to be.

Ancestry of “*sy”, step by step

StepLanguageWordMeaning
1Proto-Indo-Europeanh₁sóntsactive participle of *h₁ésti; being, existence;...
2Proto-Indo-Europeanh₁ēs-to be
Every word from Proto-Indo-European h₁ēs- →Every word from Proto-Indo-European h₁sónts →
*sy — Etymology, Origin & Word History | Etymology Explorer