Where does “Adad” come from?

Adad (English) comes from Akkadian Adad, from Akkadian addum, from Proto-Semitic *hadad-.

Ancestry of “Adad”, step by step

Step	Language	Word	Meaning
1	Akkadian	Adad	—
2	Akkadian	addum	—
3	Proto-Semitic	*hadad-	—