Where does “fonte” come from?

fonte (Old Portuguese) comes from Latin fons, from Latin fontis — genitive singular of fōns.

Ancestry of “fonte”, step by step

Step	Language	Word	Meaning
1	Latin	fons	a spring, a fountain; fresh water, spring water;...
2	Latin	fontis	genitive singular of fōns

fonte — Etymology, Origin & Word History | Etymology Explorer