Where does “ei” come from?

ei (Fala) comes from Old Portuguese eu, from Latin eō, from Proto-Italic eō, from Proto-Italic ejō, from Proto-Indo-European h₁éyti, from Proto-Indo-European h₁ey- — to go.

Ancestry of “ei”, step by step

Step	Language	Word	Meaning
1	Old Portuguese	eu	I
2	Latin	eō	to go, to fare, to move (oneself) (any kind of animate or inanimate motion: walk, ride, sail, fly, etc.)
3	Proto-Italic	eō	I go
4	Proto-Italic	ejō	—
5	Proto-Indo-European	h₁éyti	to go, to be going
6	Proto-Indo-European	h₁ey-	to go

Every word from Proto-Indo-European h₁ey- →Every word from Proto-Indo-European h₁éyti →Every word from Proto-Italic ejō →