Where does “tri-annual” come from?

tri-annual (English) comes from English tri-, from Sanskrit त्रि, from Proto-Indo-Aryan tráyas, from Proto-Indo-Iranian tráyas, from Proto-Indo-European *tréyes.

Ancestry of “tri-annual”, step by step

StepLanguageWordMeaning
1Englishtri-API; three; Used in combination with a known...
2Sanskritत्रिthree
3Proto-Indo-Aryantráyasthree
4Proto-Indo-Iraniantráyasthree
5Proto-Indo-European*tréyes—
Every word from Proto-Indo-European *tréyes →Every word from English tri- →Every word from Proto-Indo-Iranian tráyas →