Artist: George Cruikshank   /   Image source: https://www.britishmuseum.org/collection/object/P_1978-U-953
See: https://www.loc.gov/everyday-mysteries/meteorology-climatology/item/what-is-the-origin-of-the-phrase-its-raining-cats-and-dogs
for a nice explanation of the possible origin of the expression cats & dogs.
One of the possibilities it mentions is the Greek term: κατά δόξα = kata doxa = contrary to experience or belief.
Hence: It's raining unbelievably hard, as never seen before.