Where does “ianus” come from?

ianus (Latin) comes from Proto-Indo-European yeh₂- — to go; to go in, enter; to ride, to travel.

Ancestry of “ianus”, step by step

Step	Language	Word	Meaning
1	Proto-Indo-European	yeh₂-	to go; to go in, enter; to ride, to travel

ianus — Etymology, Origin & Word History | Etymology Explorer