'antepenult' originates from Latin, specifically the Late Latin word 'antepaenultimus', where 'ante-' meant 'before', 'paene' meant 'almost', and 'ultimus' meant 'last'.

'antepenult' changed from Late Latin 'antepaenultimus' through Medieval Latin and was borrowed into English (via learned/technical usage) as 'antepenult' in modern form.

Initially it meant 'before the almost-last (syllable)' — effectively 'third from the end' — and this core sense has been retained in modern usage.