What does Cassī mean in Latin?

Cassī, a proper noun, means “A Celtic tribe of Britannia, mentioned by Caesar.”.

What is the genitive of Cassī?

The genitive singular is Cassōrum. Cassī is masculine, second declension.

In classical Latin, [ˈkas.siː]; in ecclesiastical (church) Latin, [ˈkas.si].

proper noun · masculinesecond declension

Cassī m pl (genitive Cassōrum)

Classical[ˈkas.siː]Ecclesiastical[ˈkas.si]

A Celtic tribe of Britannia, mentioned by Caesar.
Trinovantibus defensis atque ab omni militum iniuria prohibitis, Cenimagni, Segontiaci, Ancalites, Bibroci, Cassi legationibus missis sese Caesari dedunt.The Trinobantes being protected and secured from any violence of the soldiers, the Cenimagni, the Segontiaci, the Ancalites, the Bibroci, and the Cassi, sending embassies, surrendered themselves to Caesar.c. 52 BCE, Julius Caesar, Commentarii de bello Gallico 5.21

Cassi, ōrum, m.,

I. a tribe of Britain, now Herts, Caes. B. G. 5, 21.