Antediluvian
Etymology
Coined in English in the seventeenth century (often credited to Sir Thomas Browne) from Latin ante, "before," and diluvium, "flood, deluge," from diluere, "to wash away." It literally means "before the flood" — the great Biblical Flood of Noah — and first described the world and its long-lived patriarchs in the era before that catastrophe. From "belonging to that unimaginably remote age" it became a wry term for anything hopelessly out of date.
adjective · /an-tee-dih-LOO-vee-uhn/
Extremely old and outdated, as if belonging to a vanished ancient age.
What it names
The word, usually half-joking, for something absurdly antiquated — antediluvian technology, antediluvian attitudes — treated as a relic of an almost prehistoric past.
Example
He still kept the accounts in a leather ledger, an antediluvian habit that baffled every new hire.
Related idea
Obsolescent describes something currently sliding out of use; antediluvian is an exaggeration, treating a thing as if it were a fossil from a world long swept away.