To be raining cats and dogs
This is a very old expression. Ancient Norse myths held that cats were able to influence the weather and that the dog was a symbol of the wind. So, " raining cats and dogs" came to be associated with severe rainstorms.
The last straw OR the straw that broke the camel’s back
This expression was first used by Charles Dickens in 1848 in his novel " Dombey & Son” It refers to the idea that although camels can carry enormous weights, but even the strongest will eventually collapse if it is overburdened. This idea is often applied to humans, too.
To knock on wood
This is a superstition that has persisted for years. In olden times, trees were believed to harbor protective spirits. To rap on a tree was to call up friendly spirit that lived there. It was believed that this spirit would protect one against impending bad luck
To get one's goat
This means to annoy or irritate someone. The saying refers to sneaky practices at the racetrack. Goats were often kept in the stalls with high-strung racehorses to keep them calm. If a goat was removed from a stall, the horse would become agitated and wouldn't race well which was quite annoying for everyone who bet on that particular horse.
To get up on the wrong side of bed
It is believed to doom a person to a day of bad luck and a bad temper. This originates from the superstition that the left side of the bed is the bad one. So getting out of bed on the left or “wrong” side is said to be unlucky
To let the cat out of the bag
It means to reveal a secret. This saying goes back to old times in England when crooked merchants would put a cat in a sack and then tell buyers that it was a pig. When the buyers got home, they would " let the cat out of the bag " and discover that they had been cheated