O / octāvus-dĕcĭmus
adjective

octāvus-dĕcĭmus

fem. octāvus-dĕcĭma · neut. octāvus-dĕcĭmum
the eighteenth
the eighteenth: pars, Vitr. 3, 3: anno aetatis, Tac. A. 13, 6.