Where did the expression "hair of the dog that bit you" come from?
When people have a hangover, they are advised to take "a hair of the same dog" the next morning. It is an allusion to an ancient notion that the burnt hair of a dog is an antidote to its bite.

Who Let the Cat Out of the Bag?

Did you ever wonder where this expression came from? In ancient times it was a trick among country folk to substitute a cat for a suckling pig and bring it to market in a bag. If anyone chose to buy the pig without examination, then the trick worked. If the person opened the sack, then he literally let the cat out of the bag, and the trick was disclosed

It's Raining Cats and Dogs!

The origin of this expression came from Northern mythology, where both animals were believed to be attendants of Odin, the storm-god. The cat symbolized down-pouring rain and the dog represented strong gusts of wind that accompany a rainstorm. A "rain of cats and dogs" is therefore a heavy rain with wind.