Where does “robus” come from?

robus (Latin) comes from Proto-Italic rouβos, from Proto-Indo-European h₁rewdʰ- — red.

Ancestry of “robus”, step by step

Step	Language	Word	Meaning
1	Proto-Italic	rouβos	—
2	Proto-Indo-European	h₁rewdʰ-	red

robus — Etymology, Origin & Word History | Etymology Explorer