Where does “fortis” come from?

fortis (Proto-Italic) comes from Proto-Indo-European bʰértis, from Proto-Indo-European bʰer- — to bear, carry.

Ancestry of “fortis”, step by step

Step	Language	Word	Meaning
1	Proto-Indo-European	bʰértis	the act of carrying, bearing
2	Proto-Indo-European	bʰer-	to bear, carry

fortis — Etymology, Origin & Word History | Etymology Explorer