Where does “Rouen” come from?

Rouen (Portuguese) comes from French Rouen, from Old French Roem, from Latin Rotomagus, from Gaulish Ratumacos, from Gaulish magos, from Proto-Celtic magos, from Proto-Indo-European meǵh₂- — big, great; great; big, stour, great.

Ancestry of “Rouen”, step by step

Step	Language	Word	Meaning
1	French	Rouen	—
2	Old French	Roem	—
3	Latin	Rotomagus	The chief city of the Veliocasses in Gallia Lugdunensis, now Rouen
4	Gaulish	Ratumacos	—
5	Gaulish	magos	field
6	Proto-Celtic	magos	plain, field
7	Proto-Indo-European	meǵh₂-	big, great; great; big, stour, great

Every word from Proto-Indo-European meǵh₂- →Every word from Proto-Celtic magos →Every word from Gaulish magos →