<span class="searchmatch">octōnī</span> inflection of octōnus: nominative/vocative masculine plural genitive masculine/neuter singular...
<span class="searchmatch">octōnīs</span> dative/ablative masculine/feminine/neuter plural of octōnus...
From the Latin octōnārius (“containing eight”), from <span class="searchmatch">octōnī</span> (“eight at a time”, “by eights”, “eight each”) + -ārius (whence the English suffix -ary). octonary...
52 BCE, Julius Caesar, Commentarii de Bello Gallico 7.73: Huius generis <span class="searchmatch">octoni</span> ordines ducti ternos inter se pedes distabant. Rows by eights of this kind...