Raining Cats and Dogs- what does it mean?

In 1600s England it was common practice to discard any waste into the streets - even dead household pets.
Once it rained so much that the now-deceased Tabbies and Fidos became buoyant and floated along the streets, thus inspiring writer Richard Brome in 1651 to record,
"it shall rain dogs and polecats.

http://www.neatorama.com/2008/12/08/the-science-behind-some-popular-phrases/