What does Islāmus mean in Latin?

Islāmus, a proper noun, means “Islam”.

What is the genitive of Islāmus?

The genitive singular is Islāmī. Islāmus is masculine, second declension.

In classical Latin, [ɪsˈɫaː.mʊs]; in ecclesiastical (church) Latin, [izˈlaː.mus].

proper noun · masculinesecond declension

Islāmus m sg (genitive Islāmī)

Classical[ɪsˈɫaː.mʊs]Ecclesiastical[izˈlaː.mus]

Islam
Quinque turrium deſcenderunt; vbi paullo inferiu eſt alia quædam porta, quam Islami noſtri, ait auctor, diuino beneficio apertã inuenerunt.1591, Johannes Löwenklau, André Wechel, Historiae musulmanae Turcorum, de monumentis ipsorum exscriptae libri XVIII., →OCLC, page 46:

Borrowed from Arabic إِسْلَام (ʔislām, literally “submission, surrender”).