From Proto-Indo-European *-tós, suffix forming possessive adjectives from nouns (compare -ātus), combined with preceding material of uncertain origin (different words may have different sources of -ī-). According to one hypothesis, some words ending in -ītus 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 crīnītus (“long-haired”), the ending -ītus 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 crīnītur in Statius is presumably based on reinterpretation of crīnītus as a participle.
-ītus (feminine -īta, neuter -ītum); first/second-declension suffix
-itus (not comparable)
-ītus