Where does “надвае” come from?

надвае (Belarusian) comes from Belarusian на, from Proto-Slavic na, from Proto-Indo-European h₂en- — on, onto.

Ancestry of “надвае”, step by step

надвае traces back through two separate lines of ancestry.

via Belarusian на

StepLanguageWordMeaning
1Belarusianнаon, onto
2Proto-Slavicnaon; concerning; onto
3Proto-Indo-Europeanh₂en-on, onto

via Belarusian двое

StepLanguageWordMeaning
1Belarusianдвоеtwo; two of.
2Proto-Slavic*dъvojьtwo
Every word from Proto-Indo-European h₂en- →Every word from Proto-Slavic na →Every word from Belarusian на →