Where does “vassa” come from?

vassa (Pali) comes from Sanskrit वर्ष, from Proto-Indo-European h₁wers-ó-m, from Proto-Indo-European h₁wers- — to rain.

vassa (Pali): rain; year

Definitions

Step	Language	Word	Meaning
1	Sanskrit	वर्ष	rain, raining, a shower; the rains, rainy season;...
2	Proto-Indo-European	h₁wers-ó-m	—
3	Proto-Indo-European	h₁wers-	to rain

vassa — Etymology, Origin & Word History | Etymology Explorer