ferus

Ultimately from Proto-Indo-European *ǵʰweh₁r-^[1] or *ǵʰwē̆r- (“wild animal”).^[2] The intermediate steps are uncertain:

• Possibly from Proto-Italic *xʷeros, from earlier *ǵʰweh₁rós, with PIE *-ē- becoming Proto-Italic *-e- because of a shortening of PIE unaccented long vowels before a resonant (a version of a sound change called "Dybo's Law"). This change is also supposed to explain the short vowel in vir.^[1][3] In contrast to vir (from *wiros), this term retained the final -rus, possibly due to analogical influence from the form fera.^[1]
• Alternatively, Sihler considers the adjective a back-formation from the feminine noun fera (“wild animal, wild beast”), and treats it as possible that Latin continues an original short e-grade of the PIE root.^[2]

• (Classical Latin) IPA^(key): [ˈfɛ.rʊs]
• (modern Italianate Ecclesiastical) IPA^(key): [ˈfɛː.rus]

ferus (feminine fera, neuter ferum); first/second-declension adjective

1. wild, savage, fierce, cruel

   Synonyms: trux, ferōx, atrōx, violēns, immānis, efferus, crūdēlis, silvāticus, ācer, acerbus, sevērus

   Antonyms: mītis, tranquillus, misericors, placidus, quietus, clemens

   • 8 CE, Ovidius, Fasti 4.107–108:

     prīma ferōs habitūs hominī dētrāxit: ab illā
     vēnērunt cultūs mundaque cūra suī.

     [Venus] first divested men of savage habits: from her
     came fancy attire and clean care of oneself.

2. uncivilized, uncultivated

   Synonym: barbaricus

3. untamed, rough

First/second-declension adjective.

• efferus

• ferōx

ferus m (genitive ferī); second declension

1. wild animal

Second-declension noun.