What does coaeternus mean in Latin?

coaeternus, a adjective, means “coeternal”.

Where does coaeternus come from?

Proto-Indo-European *ḱe? → Proto-Indo-European *ḱóm → Proto-Italic *kom → Proto-Italic *kom- → Latin coaeternus.

adjectivefirst/second-declension adjective

coaeternus (feminine coaeterna, neuter coaeternum)

Ecclesiastical[ko.eˈtɛr.nus]

singular
form	masculine	feminine	neuter
nominative	coaeternus	coaeterna	coaeternum
genitive	coaeternī	coaeternae	coaeternī
dative	coaeternō	coaeternae	coaeternō
accusative	coaeternum	coaeternam	coaeternum
ablative	coaeternō	coaeternā	coaeternō
vocative	coaeterne	coaeterna	coaeternum

plural
form	masculine	feminine	neuter
nominative	coaeternī	coaeternae	coaeterna
genitive	coaeternōrum	coaeternārum	coaeternōrum
dative	coaeternīs	coaeternīs	coaeternīs
accusative	coaeternōs	coaeternās	coaeterna
ablative	coaeternīs	coaeternīs	coaeternīs
vocative	coaeternī	coaeternae	coaeterna

cŏ-aeternus, a, um, adj.,

I. coeternal (eccl. Lat.), Tert. adv. Herm. 11; Hier. Ep. 16, n. 4 al.

Latin aeternus Latin coaeternus From con- + aeternus.