'postdiluvian' originates from Latin, specifically the prefix 'post' and the word 'diluvium', where 'post' meant 'after' and 'diluvium' meant 'a flood'.

'postdiluvian' changed from Late Latin 'postdiluvianus' and passed into English via Middle English/early modern English forms and eventually became the modern English word 'postdiluvian'.

Initially, it meant 'after the Flood' (literally referring to the biblical Flood), but over time it also acquired the figurative meaning 'very old‑fashioned' or 'antiquated'.