What does octāvusdecimus mean in Latin?

octāvusdecimus, a numeral, means “eighteenth”.

numeralfirst/second-declension numeral with a first/second-declension numeral

octāvusdecimus

octāvusdecimus (feminine octāvadecima, neuter octāvumdecimum)

singular
form	masculine	feminine	neuter
nominative	octāvusdecimus	octāvadecima	octāvumdecimum
genitive	octāvīdecimī	octāvaedecimae	octāvīdecimī
dative	octāvōdecimō	octāvaedecimae	octāvōdecimō
accusative	octāvumdecimum	octāvamdecimam	octāvumdecimum
ablative	octāvōdecimō	octāvādecimā	octāvōdecimō
vocative	octāvedecime	octāvadecima	octāvumdecimum

plural
form	masculine	feminine	neuter
nominative	octāvīdecimī	octāvaedecimae	octāvadecima
genitive	octāvōrumdecimōrum	octāvārumdecimārum	octāvōrumdecimōrum
dative	octāvīsdecimīs	octāvīsdecimīs	octāvīsdecimīs
accusative	octāvōsdecimōs	octāvāsdecimās	octāvadecima
ablative	octāvīsdecimīs	octāvīsdecimīs	octāvīsdecimīs
vocative	octāvīdecimī	octāvaedecimae	octāvadecima

octāvus-dĕcĭmus, a, um, adj.,

I. the eighteenth: pars, Vitr. 3, 3: anno aetatis, Tac. A. 13, 6.