Where does “Ιανουάριος” come from?

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

Ancestry of “Ιανουάριος”, step by step

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