Where does “rro” come from?

rro (Proto-Brythonic) comes from Proto-Celtic ɸro, from Proto-Indo-European pró, from Proto-Indo-European per-, from Proto-Indo-European pr̥tós — passed , crossed.

Ancestry of “rro”, step by step

Step	Language	Word	Meaning
1	Proto-Celtic	ɸro	—
2	Proto-Indo-European	pró	toward, leading to
3	Proto-Indo-European	per-	before, in front; first; to go through
4	Proto-Indo-European	pr̥tós	passed , crossed

Words derived from “rro”

rhy
crybwyll
rhawch
crybwylliad

Every word from Proto-Indo-European pr̥tós →Every word from Proto-Indo-European per- →Every word from Proto-Indo-European pró →