出典:Wiktionary
Probably a form of -tus (adjective-forming suffix) used as a neuter substantive representing location. This may have originated from a reanalysis of ante-classical olētum (“olive yard”) from ole(a) (“olive tree”) + -tum to ol(ea) + -ē̆tum. Compare to other neuters representing location: -ārius ~ -ārium, -īlis ~ -īle.