Where does “ienuārius” come from?

ienuārius (Vulgar Latin) comes from Latin iānuārius, from Latin -ārius, from Proto-Italic *-āzios.

Ancestry of “ienuārius”, step by step

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