Raining Cats And Dogs

It's really been coming down in buckets off and on all day. I had to go out to drop off paintings at a group show I'm participating in. The rain had let up, so I stopped off at the grocery store. By the time I got out, with my $100. plus purchases, it was pouring again. The fifteen minute drive home took almost a half an hour. I just kept thinking about that saying , " it's raining cats and dogs". What a fun painting it would be to create! It also got me wondering what the origins of the saying might actually be. Any ideas anyone?