What does Iōniacus mean in Latin?

Iōniacus, a adjective, means “Ionian”.

How do you pronounce Iōniacus?

In classical Latin, [i.oːˈni.a.kʊs]; in ecclesiastical (church) Latin, [i.oˈniː.a.kus].

adjectivefirst/second-declension adjective

Iōniacus (feminine Iōniaca, neuter Iōniacum)

Classical[i.oːˈni.a.kʊs]Ecclesiastical[i.oˈniː.a.kus]

singular
form	masculine	feminine	neuter
nominative	Iōniacus	Iōniaca	Iōniacum
genitive	Iōniacī	Iōniacae	Iōniacī
dative	Iōniacō	Iōniacae	Iōniacō
accusative	Iōniacum	Iōniacam	Iōniacum
ablative	Iōniacō	Iōniacā	Iōniacō
vocative	Iōniace	Iōniaca	Iōniacum

plural
form	masculine	feminine	neuter
nominative	Iōniacī	Iōniacae	Iōniaca
genitive	Iōniacōrum	Iōniacārum	Iōniacōrum
dative	Iōniacīs	Iōniacīs	Iōniacīs
accusative	Iōniacōs	Iōniacās	Iōniaca
ablative	Iōniacīs	Iōniacīs	Iōniacīs
vocative	Iōniacī	Iōniacae	Iōniaca

ĭōnĭăcus, a, um, adj., = Ἰωνιακός,

Borrowed from Ancient Greek Ἰωνῐᾰκός (Iōnĭăkós).