Where does “יי” come from?

יי (Hebrew) comes from Hebrew יאהדונהי, from Hebrew יהוה, from Proto-Semitic *haway-.

Ancestry of “יי”, step by step

Step	Language	Word	Meaning
1	Hebrew	יאהדונהי	—
2	Hebrew	יהוה	YHWH, Yahweh, Jehovah; God; Yahweh
3	Proto-Semitic	*haway-	—