'post-Flood' is formed from the prefix 'post-' from Latin 'post' meaning 'after' combined with 'Flood' from Old English 'flōd' meaning 'flood, rising water'.

'post-' entered English via Late Latin and was used in compounds in Middle English; 'flood' changed from Old English 'flōd' through Middle English 'flod' to modern English 'flood', and the compound 'post-Flood' developed by combining the prefix with the noun.

Originally a literal combination meaning 'after a flood (waters)', the term came to be used specifically for the period after the Biblical Flood when capitalized ('Flood'), while lowercase uses refer to any flood event.