-itus

Etymology 1
From, suffix forming possessive adjectives from nouns (compare ), combined with preceding material of uncertain origin (different words may have different sources of -ī-). According to one hypothesis, some words ending in are "decasuative" formations derived from former inflected noun forms that ended in -ī, such as the instrumental singular form of i-stem nouns or the genitive singular of o-stem nouns. For example, in, the ending may be from instrumental *-ih₁ + *-to-. But this type of derivation is disputed. Other words may have been formed by analogy. It is often difficult to distinguish the adjectival ending from the fourth-conjugation participial ending, and in some cases fourth-conjugation verbs may have been derived from original adjectives by back-formation; for example, the occurrence of the finite verb form in Statius is presumably based on reinterpretation of  as a participle.