We often quote phrases we have heard or read, but most of the time we may not know how the term even originated. The Science Behind Some Popular Phrases explores some commonly used phrases.
Mad as a Hatter: Today we know enough to keep clear of mercury, but hat makers once used it to make the brims of hats. When absorbed through the skin, it could wreak havoc on the nervous system: tremors, fatigue, not to mention behavioral dysfunction - that is, crazy behavior. Just think of Lewis Carroll's Mad Hatter from Alice's Adventures in Wonderland.Raining Cats and Dogs: 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."
Read more explanations here.