'pre-Flood' originates from the Latin prefix 'pre-' (from 'prae') meaning 'before', combined with the English word 'Flood' which ultimately comes from Old English 'flod' meaning 'flood' or 'torrent'.

'pre-Flood' was formed in Modern English by attaching the Latin-derived prefix 'pre-' to the English noun 'Flood' (capitalized when referring to the biblical event), producing the compound used to denote the time before that Flood.

Initially it would have meant simply 'before a flood'; over time it has been used especially for the biblical Flood and metaphorically to indicate something very ancient (similar to 'antediluvian').