Where does “ad fontes” come from?

ad fontes (Latin) comes from Latin ad, from Proto-Italic ad, from Proto-Indo-European h₂éd — to, at.

Ancestry of “ad fontes”, step by step

Step	Language	Word	Meaning
1	Latin	ad	toward, to; up to; near, by, close by, at, to
2	Proto-Italic	ad	toward, to, on, up to, for
3	Proto-Indo-European	h₂éd	to, at

ad fontes — Etymology, Origin & Word History | Etymology Explorer