The adjectival suffixes -ārius, -tōrius, and -sōrius denote belonging to a group qualified by the implied content of the correspond root. Effectively, these adjectives are formed by the addition to -ius to the adjectival root -āris or the nominal root -or. Let’s build a few examples.
- bellum (war) —> bellātor (warrior) —> bellātōrius, -a, -um (of warriors)
- pecūlium (property) —> pecūliāris, -e (one’s own) —> pecūliārius, -a,- um (of private property)
- meritō (to earn) —> meritus (earned) —> [meritor] (earner) —> meritōrius, -a, -um (profitable, esp. related to prostitution)
- extrā (outside) —> [extrāris, e] (foreign) —> extrārius, -a, -um (of foreigners)
A few things to notice about this pattern: (i) the original base of the adjective can be just about anything—noun, adjective, verb, adverb—but the penultimate word is always a noun or an adjective. That said, (ii) the penultimate noun or adjective is not always extant in Latin; note the [brackets]. Finally, note that (iii) this set of adjectives is often theoretical—rēs bellatōriae (matters of warriors)and rēs extrāriae (matters of foreigners) probably cover half the total appearances of those two adjectives.