Late 19th century; earliest use found in The Athenaeum. From classical Latin octōnārius a verse of eight feet + octonarius and slightly earlier octonare.
Definition of octonarian in US English:
octonarian
adjectiveˌɒktə(ʊ)ˈnɛːrɪən
Prosody rare
Consisting of eight feet.
Origin
Late 19th century; earliest use found in The Athenaeum. From classical Latin octōnārius a verse of eight feet + octonarius and slightly earlier octonare.