What does Trēvir mean in Latin?

Trēvir, a proper noun, means “A member of the Treveri, ancient Celtic tribe in Gallia Belgica whose chief town was Augusta Treverorum”.

What is the genitive of Trēvir?

The genitive singular is Trēvirī. Trēvir is masculine, second declension.

In classical Latin, [ˈtreː.wɪr]; in ecclesiastical (church) Latin, [ˈtrɛː.vir].

proper noun · masculinesecond declension

Trēvir m (genitive Trēvirī)

Classical[ˈtreː.wɪr]Ecclesiastical[ˈtrɛː.vir]

A member of the Treveri, ancient Celtic tribe in Gallia Belgica whose chief town was Augusta Treverorum
Diebus circiter XV, quibus in hiberna ventum est, initium repentini tumultus ac defectionis ortum est ab Ambiorige et Catuvolco; qui, cum ad fines regni sui Sabino Cottaeque praesto fuissent frumentumque in hiberna comportavissent, Indutiomari Treveri nuntiis impulsi suos concitaverunt subitoque oppressis lignatoribus magna manu ad castra oppugnatum venerunt.About fifteen days after they had come into winter-quarters, the beginning of a sudden insurrection and revolt arose from Ambiorix and Catuvolcus, who, though they had met with Sabinus and Cotta at the borders of their kingdom, and had conveyed corn into our winter-quarters, induced by the messages of Indutiomarus, one of the Treveri, excited their people, and after having suddenly assailed the soldiers engaged in procuring wood, came with a large body to attack the camp.c. 52 BCE, Julius Caesar, Commentarii de bello Gallico 5.26

See Treveri