Raining cats and dogs

Of course,  this idiom means that it is raining very heavily! But how did this come about? The short answer is that nobody knows !! One guess is that it is a corruption of the Greek words 'cata doxa' , which means 'beyond belief'. Another guess is that it is from the Norse belief that cats could influence the weather and that dogs were a signal of storms.
Maybe the rain falling on the roof was so loud that it sounded as if cats and dogs were fighting on the roof!!! Who knows?
The phrase was first published in  book of poems in 1661, so it has been around for a while😀