What does triennis mean in Latin?

triennis, a adjective, means “three-year-old; of three years, lasting three years”.

How do you pronounce triennis?

In classical Latin, [triˈɛn.nɪs]; in ecclesiastical (church) Latin, [triˈɛn.nis].

adjectivethird-declension two-termination adjective

triennis (neuter trienne)

Classical[triˈɛn.nɪs]Ecclesiastical[triˈɛn.nis]

two terminationthree-year-old
Dixitque ad eum : Ego Dominus qui eduxi te de Ur Chaldæorum ut darem tibi terram istam, et possideres eam. At ille ait : Domine Deus, unde scire possum quod possessurus sim eam ? Et respondens Dominus : Sume, inquit, mihi vaccam triennem, et capram trimam, et arietem annorum trium, turturem quoque et columbam.Vulgata—Genesis 15.9
two terminationof three years, lasting three years

singular
form	masculine	feminine	neuter
nominative	triennis	triennis	trienne
genitive	triennis	triennis	triennis
dative	triennī	triennī	triennī
accusative	triennem	triennem	trienne
ablative	triennī	triennī	triennī
vocative	triennis	triennis	trienne

plural
form	masculine	feminine	neuter
nominative	triennēs	triennēs	triennia
genitive	triennium	triennium	triennium
dative	triennibus	triennibus	triennibus
accusative	triennīs triennēs	triennīs triennēs	triennia
ablative	triennibus	triennibus	triennibus
vocative	triennēs	triennēs	triennia

trĭennis, e, adj. tres-annus,

I. three years old: vacca, Vulg. Gen. 15, 9.

From tri- (“three”) + annus (“year”) + -is (suffix forming compound adjectives).