-ius

• (Classical Latin) IPA^(key): /i.us/,
• (modern Italianate Ecclesiastical) IPA^(key): /i.us/, (stressed on the antepenult)

From Old Latin -ios, from Proto-Italic *-jos, from Proto-Indo-European *-yós. Cognate with Ancient Greek -ῐος (-ios).

-ius (feminine -ia, neuter -ium); first/second-declension suffix

1. forming adjectives from nouns
2. found as an ending of most Classical Latin 'nomina gentilicia' (family names or surnames, shared by the members of a gens)
3. (New Latin) suffix for Latinizing surnames

The suffix -ius is added to a noun to form an adjective indicating "made of" or "belonging to" that noun.

Examples:

‎pater (“father”) + ‎-ius → ‎patrius (“paternal”)

‎rēx (“king, ruler”) + ‎-ius → ‎rēgius (“kingly, royal”)

‎uxor (“wife”) + ‎-ius → ‎uxōrius (“uxorious”)

‎papȳrus (“papyrus”) + ‎-ius → ‎papȳrius (“made of papyrus”)

In taxonomics,^[1] out of analogy with ancient Roman nomina gentilicia, this suffix is added to surnames ending in a consonant other than the ending -er and, sometimes, replacing a mute final -e.

Examples:

‎French Descartes + ‎-ius → ‎Cartesius

‎German Leibniz + ‎-ius → ‎Leibnitius

‎German Schmalz + ‎-ius → ‎Smalcius

‎English Shakespeare + ‎-ius → ‎Shakespeārius

First/second-declension adjective.

• -āceus
• -āneus
• -ticus

• -ium

• “-ius” on page 986/1 of the Oxford Latin Dictionary (1st ed., 1968–82)

See -ior (suffix forming adjectives’ comparative degrees).

-ius

1. nominative/accusative/vocative neuter singular of -ior

See -ō (suffix forming adverbs).

-ius

1. comparative degree of adverbs ending in -ō

From Proto-Italic *-osjos, itself from Proto-Indo-European *-ósyo (genitive case suffix) secondarily marked with the genitive *-s.

-ius

1. the regular genitive singular suffix for most pronouns

   quī, cuius

   hic, huius

   ūnus, ūnī̆us

   alter, alterī̆us (alongside alterī, alterae)

Like 3d- and 4th-declension, and unlike 1st- and 2nd-declension forms, has one form for all genders. A gendered adjective option also exists for cuius.