Cats And Dogs Idiom. Alternatively, `cats and dogs' could be a corruption or misunderstanding of the greek word `catadupe', meaning `waterfall', so the expression would originally have been `it's raining like a waterfall'. There is also the airplane pilot's description of truly bad weather when driving a car:
The correct idiom is “fight like cats and dogs”. Raining cats and dogs is an idiom for rains a lot. Dog in the manger throw to the wolves
♦ apr 15 '13 at 10:50
This fun cat idiom has been used for centuries. Fighting like cats and dogs is another common cat idiom you might hear which references canines. That at least is a plausible theory. This is something that you say when it is raining very, very heavily.