What's the difference between an idiot and an idiom? The last letter, of course. Well, that and the meaning. Lest you appear to be an idiot, here is an education on the roots of some common idioms. . .

Raining Cats and Dogs
This saying goes way back to the 1600's. The thatched roofs on 17th century homes also housed small mammals such as rodents and possibly a cat or two. After a heavy rainfall, those that had crawled up there to die were washed into overflowing gutters and kablam. If your umbrella wasn't heavy duty, you could literally be smacked in the head with a dead cat.

Wrong Side of the Tracks
When steam engines used to roll through town, black smoke and soot billowed out of the smokestack. The wind generally blew that smoke in the same direction. Would you want to live there? Nope. Most people didn't. So the poorest of the poor took up residence on the nasty sooty smoky side, hence the phrase.

Don't Throw the Baby Out 
With the Bath Water
Families used to share bath tubs. Yeah. I know. Get over the eew factor and you'll be fine. In pre-running water days, the tub would be filled and the head of the house got the first bath, followed by the next oldest and so on and so on. By the time it was the baby's turn, that water was pretty dirty, so it was a good idea to make sure the kid wasn't still in it by the time you tossed it out of the house.

Mad as a Hatter
Felt hats used to be made by using mercury in the process. Worked great on fabric. Not so much on brain cells. Mercury poisoning causes tremors, brain damage, tooth loss, slurred speech and other freakish side effects. Think of it as a perk of the job.

To Be Stumped
This is another train transportation era phrase. Land had to be cleared to lay down tracks. When workers came across a tree stump, it caused a dilemma, or in other words, to be stumped.