Antediluvian
Definition: old
This technically means “before the Great Flood.” “Diluvial” means “related to a flood.”
But, over the years, it’s become a synonym for “old” or “ancient,” when used to refer to non-historical things.
“Diluvian” means “related to or brought about by a flood” and “ante” is the prefix for “before.”
Why I Looked It Up
In Rogue Protocol:
On three antediluvian computer monitors, green-blue spirals drill ever-deeper.
Update
Added on
I found it twice in Conspiracies and Secret Societies. In a discussion of a supposedly ancient airship:
A common theme is that certain men of genius in ancient Egypt and Persia were given access to the records of the advanced technologies of the antediluvian world.
The book also mentioned a book published in 1882 by Ignatious L. Donnelly called Atlantis: The Antediluvian World.