Where does “fulgus” come from?

fulgus (Vulgar Latin) comes from Latin fulgur, from Proto-Italic folgos — brightness.

Ancestry of “fulgus”, step by step

Step	Language	Word	Meaning
1	Latin	fulgur	lightning, a flash of lightning; thunderbolt
2	Proto-Italic	folgos	brightness

fulgus — Etymology, Origin & Word History | Etymology Explorer