Where does “चय” come from?

चय (Sanskrit) comes from Sanskrit चि, from Proto-Indo-European kʷey- — to pay; to avenge.

Ancestry of “चय”, step by step

Step	Language	Word	Meaning
1	Sanskrit	चि	—
2	Proto-Indo-European	kʷey-	to pay; to avenge

चय — Etymology, Origin & Word History | Etymology Explorer