What does Arēus mean in Latin?

Arēus, a proper noun, means “a river in Bithynia, mentioned by Pliny”.

What is the genitive of Arēus?

The genitive singular is Arēī. Arēus is masculine, second declension.

In classical Latin, [aˈreː.ʊs]; in ecclesiastical (church) Latin, [aˈrɛː.us].

proper noun · masculinesecond declension

Arēus m sg (genitive Arēī)

Classical[aˈreː.ʊs]Ecclesiastical[aˈrɛː.us]

Ărēus, a, um, adj., = Ἄρειος,

I. pertaining to Mars: judicium, the Areopagus, Tac. A. 2, 55; v. Areopagus.