Students often ask me, as I have previously stated in another post, the origins of words and phrases because I love reading about the stories behind them. One popular question I get is “why do people say it’s raining cats and dogs?” (just like in the picture to the right).
There is no definitive answer for it, though there are a number of theories (including a few here and here). However as I have read in more than one place, the most plausible origin for the phrase is that English streets were filthy; repugnant is probable a better word to describe the offal, refuse, and putrid debris carried along the streets after heavy rains. People would not have been surprised to see the carcasses of animals float by them. In fact, Jonathan Swift described such a disgusting picture in a poem (check out the last two lines).
The poem and/or the common sights of death floating along the streets may have given rise to the saying. Or perhaps there is another explanation that you may know?