Where does “iunior” come from?

iunior (Latin) comes from Latin iuvenior, from Latin iuvenis, from Proto-Italic *juwō.

Ancestry of “iunior”, step by step

Step	Language	Word	Meaning
1	Latin	iuvenior	younger, more youthful
2	Latin	iuvenis	young; A youth, a young man, young woman, young...
3	Proto-Italic	*juwō	—

iunior — Etymology, Origin & Word History | Etymology Explorer