Where does “Ἰανουάριος” come from?

Ἰανουάριος (Ancient Greek) comes from Latin iānuārius, from Latin -ārius, from Proto-Italic *-āzios.

Ancestry of “Ἰανουάριος”, step by step

Step	Language	Word	Meaning
1	Latin	iānuārius	—
2	Latin	-ārius	—
3	Proto-Italic	*-āzios	—

Ἰανουάριος — Etymology, Origin & Word History | Etymology Explorer