What does Cyllēnius mean in Latin?

Cyllēnius, a noun, means “An epithet of the god Mercury; Cyllenian”.

What is the genitive of Cyllēnius?

The genitive singular is Cyllēniī. Cyllēnius is masculine, second declension.

In classical Latin, [kylˈlɛ.ni.ʊs]; in ecclesiastical (church) Latin, [t͡ʃilˈlɛː.ni.us].

noun · masculinesecond declension

Cyllēnius m sg (genitive Cyllēniī or Cyllēnī)

Classical[kylˈlɛ.ni.ʊs]Ecclesiastical[t͡ʃilˈlɛː.ni.us]

RomanAn epithet of the god Mercury; Cyllenian
hic primum paribus nitens Cyllenius alis constitit; [...]here first on even wings did shining Mercury stand29 BCE – 19 BCE, Vergilius, Aeneis 4.252–253

Charlton Lewis's shorter dictionary for students: the same scholarship in plain, modern definitions.

Cyllēnius adj.,

of Cyllene: proles, i. e. Mercury, V.: ignis, the planet Mercury, V.—As subst m., Mercury, V., O.

From Ancient Greek Κυλλήνιος (Kullḗnios), from Κυλλήνη (Kullḗnē), which in legend is described as the birthplace of the god.