What does Cornubia mean in Latin?

Cornubia, a proper noun, means “Cornwall (a peninsula, county, and cultural region in southwestern England)”.

What is the genitive of Cornubia?

The genitive singular is Cornubiae. Cornubia is feminine, first declension.

proper noun · femininefirst declension

Cornubia f sg (genitive Cornubiae)

Medieval LatinfemininesingularCornwall (a peninsula, county, and cultural region in southwestern England)
Quando proféctus freram - Usque díram Domnóniam - Pér caréntem Cornúbiam...When I had set forth - toward dire Devon - through Cornwall lacking...ca. 670 CE - ca. 710 CE, Aldhelmus Malmesberiensis, Aldhelmi Carmen Rhythmicum
Bellum Heyl apud Cornubiam.The battle of Hayle among Cornwall.(Can we date this quote?), Annales Cambriae B:

From Proto-Brythonic *Körnɨw + -ia, with the common Medieval confusion of b and v.