Some of these aren't 100% true (I'll let you decide which ones) but it will make you think twice before using these phrases at will.....
Rule of Thumb:
An old English law declared that a man could not beat his wife with a stick any larger than the diameter of his thumb.
Saved by the Bell:
When our ancestors realized that they were burying a great deal of people before their time had actually come, they came up with a solution. They tied a string onto the "dead" person's hand, buried them, and tied the other end of the string to a bell and then tied it to nearby tree branch. If the person revived enough to ring the bell, their survivors would rush out and dig them up. Hence... "saved by the bell" (Ewww!!!)
Bite the Bullet:
When engaged in war there are times when emergency surgery is needed: Legs have to come off or deeply-buried bullets need to come out. And sometimes, there's no time for anesthesia when the Nazis are bearing down.
So, rather then stabbing a patient in the arm to distract him from the saw going through his foot, the surgeon would supposedly shove a bullet in his mouth and ask him to bite down. Of course, you could use a belt or shirt but even in the throes of death it's important for a man to look like a badass. Thus, "Bite the bullet."
Raining Cats & Dogs:
In the 1500s human beings had the pleasure of living in homes with thatched roofs. In these strange times, humans kept their pets outside. The animals would keep themselves warm in the little nooks in the thatching on the roofs and store their food up there for a rainy day.
When an especially rainy day did come along, the animals would either get washed off of the roof or would come leaping down looking for better cover. The story goes that the townsfolk would look out their window, see pets falling from the sky, and proclaim it to be "raining cats and dogs." Then they would probably burn a witch or something.
As Mad as a Hatter: (this is one of my Mom's favs)
There is a number of theories about the root of this similie. Perhaps the most intriguing, and also plausible, was offered in "The Journal of the American Medical Association". Mercury used to be used in the manufacture of felt hats, so hatters, or hat makers, would come into contact with this poisonous metal a lot. Unfortunately, the effect of such exposure may lead to mercury poisoning, one of the symptoms of which is insanity.
It is proverbial that crocodiles cry like a person in distress to lure men close enough to snatch and devour them, then shed tears over the fate of their victim. References to this proverbial belief are found in ancient Greek and Latin literature.
Medieval physicians believed that the secretions of a frog could cure a cough if they were coated on the throat of the patient. The frog was placed in the mouth of the sufferer and remained there until the physician decided that the treatment was complete.