LATIN DICTIONARYLexicon Latīnum
proper noun · masculinesecond declension

Posīdōnius

Posīdōnius m sg (genitive Posīdōniī or Posīdōnī)
Classical[pɔ.siːˈdoː.ni.ʊs]Ecclesiastical[po.s̬iˈdɔː.ni.us]

Meaning

  1. a male given name from Ancient Greek — famously held by
  2. Posidonius, an Ancient Greek polymath of the Hellenistic period.

Declension

declension
formsingularplural
nominativePosīdōnius—
genitivePosīdōniī
Posīdōnī		—
dativePosīdōniō—
accusativePosīdōnium—
ablativePosīdōniō—
vocativePosīdōnī—

Pŏsīdōnĭus, ii, m., = Ποσειδώνιος,
I. a celebrated Stoic at Rhodes, a disciple of Panœtius and instructor of Cicero, Cic. Att. 2, 1, 2; id. Off. 3, 2, 8; id. N. D. 1, 3, 6; id. Div. 1, 3, 6 al.

Origins

Borrowed from Ancient Greek Ποσειδώνιος (Poseidṓnios).

Quick answers

What does Posīdōnius mean in Latin?
Posīdōnius, a proper noun, means “a male given name from Ancient Greek — famously held by”.
What is the genitive of Posīdōnius?
The genitive singular is Posīdōniī. Posīdōnius is masculine, second declension.
How do you pronounce Posīdōnius?
In classical Latin, [pɔ.siːˈdoː.ni.ʊs]; in ecclesiastical (church) Latin, [po.s̬iˈdɔː.ni.us].