Where does “Каинъ” come from?

Каинъ (Old Church Slavonic) comes from Ancient Greek Κάϊν, from Biblical Hebrew קַיִן.

Ancestry of “Каинъ”, step by step

Step	Language	Word	Meaning
1	Ancient Greek	Κάϊν	—
2	Biblical Hebrew	קַיִן	—

Каинъ — Etymology, Origin & Word History | Etymology Explorer