Answer:

Antediluvian

Strictly speaking, "Antediluvian" means before The Flood depicted in the Old Testament Book of Genesis.
In modern parlance, it used to describe something which is very old, and out of date.
A sentence:

The antediluvian drains were clogged so the rain flooded the streets.

First answer by Peterc14. Last edit by Peterc14. Contributor trust: 1215 [recommend contributor recommended]. Question popularity: 1 [recommend question].