Where does “весна” come from?

весна (Old East Slavic) comes from Proto-Slavic vesna, from Proto-Balto-Slavic was(e)r, from Proto-Indo-European wósr̥ — spring.

Ancestry of “весна”, step by step

StepLanguageWordMeaning
1Proto-Slavicvesnaspring
2Proto-Balto-Slavicwas(e)rspring
3Proto-Indo-Europeanwósr̥spring
Every word from Proto-Indo-European wósr̥ →Every word from Proto-Balto-Slavic was(e)r →Every word from Proto-Slavic vesna →