To let the cat out of the bag ------------------------------ This idiom was probably originated in English marketplaces many years ago. Traders would put a cat in a bag and would try to deceive possible customers by saying it was a pig. People would buy the bag without close inspection of the contents, but then they would let the cat out of the bag and realize that they had been deceived, which is the meaning it has nowadays.
To be born with a silver spoon in one's mouth -------------------------------------------- This expression is used to refer to a member of a wealthy family. It was probably originated from an old custom whereby godparents used to present the child at the christening with a silver spoon.
Never look a gift-horse in the mouth ----------------------------------- This is an old saying and it's related to the way that a horse's age is calculated: by looking at its teeth. The expression now means that you should accept a received gift without trying to find a fault in it.
To upset the apple cart ------------------------ This old expression probably derives from the way that apples were taken to market. The roads were rocky so if the wheel stuck, the whole load of apples would tumble. Nowadays, the meaning is to disrupt a plan or arrangement.
Dressed to the nines -------------------- This may come from an alteration of "dressed to the eyes", which was written in Old English as "to then eyne". The letter n of then was removed and moreover, eyne was changed to nine. Another explanation is that the number ten is considered the ultimate point of perfection. Therefore, if someone is dressed to the nines, he must look really smart.
It's raining cats and dogs --------------------------- This is a very old expression linked to the ancient beliefs of sailors and also to Norse mythology. Cats were associated with heavy rain and dogs with storms and wind. Therefore, this expression was used to refer to severe rainstorms. Nowadays, it has become old-fashioned.
To talk/speak of the devil -------------------------- The original form of the proverb was 'talk of the devil and he's sure to appear', which was already used in the 17th century. Today, only the first part of the phrase remains and is used to refer to someone who unexpectedly appears when you have just been talking about him.
To catch someone red-handed -------------------------- This meaning of this idiom is "to find someone when he's doing something wrong". It alludes to the discovery of the murderer so soon after committing the crime that blood is still on his hands.
Can't hold the candles --------------------- This idiom originated in the times when there was no electricity and candles were used instead. If a master needed light, he would employ an apprentice to hold the candle for him. Therefore, if someone was not fit to hold it or couldn't hold it to somebody else, he was considered inferior to the other. This is the meaning used nowadays. Notice that it's followed by the preposition to (can't hold the candles to somebody).
Tell it to the marines ---------------------- This expression is used to show disbelief at something that has just been said. It comes from the fact that in the past marines were considered stupid or innocent by regular sailors. They would believe anything they were told, even if it was something foolish or false. That's why when you doubt about what someone is saying you can yell at him this idiom.
To bury the hatchet ------------------- This comes from the American custom of burying hawks and other weapons as a sign that hostilities between the American Indians and the White had ended. Nowadays, this idiom is used to refer to coming to peaceful terms with an opponent.
The black sheep ----------------- Somebody is called the black sheep of the family when he/she is regarded as the odd one out, the one who brings disgrace to the rest. This expression comes from the fact that a black sheep is considered the outcast of any flock, perhaps because its dark colour frightens the white sheep, but also because the black fleece cannot be sold for wool, being of no use to the shepherd.
A white elephant ---------------- White elephants, also known as albino elephants, were considered holy in ancient times, specially in some Asian countries. To keep these animals was very expensive, because it was necessary to give them special food, and also to give access to the people who wanted to worship them. In Thailand, if a king was dissatisfied with someone, he would give him a white elephant, which would surely make that person lose all his money. Nowadays, this expression is used to refer to something that costs a lot of money to maintain but is useless.
A cock and bull story --------------------- This expression originated in the 18th century, at Stony Stratford in Buckinghamshire, England. That village had two small inns, one called 'The Cock' and the other called 'The Bull'. Coaches that would go from London to Birmingham and vice versa usually stopped there to change horses, at one inn or the other. Travellers told their own stories there, which in general were exaggerated and funny. Today this expression means a silly story or a excuse that nobody believes.
Once in a blue moon ------------------- This expression means "very rarely". The "blue moon" refers to the second full moon in the same calendar month, something that occurs not very often, almost every two years and a half, to be exact. Well, it occurred in July 2004, when there was a full moon on July 2nd and on July 31st. The second one is called "blue moon". This expression also originates from the fact that the moon is never blue, with the exception of rare occasions when there is lot of dust in the atmosphere, like during the eruption of the Krakatoa volcano in 1883 and the forest fires in Canada in 1951.
To let off some steam --------------------- As you know, in steam heating systems and steam engines (like steam locomotives), boilers are used. They contain water that is heated by burning some fuel and then, the heated water turns to steam. This steam generates a lot of pressure in the boiler. If there's too much pressure, the boiler may explode. That's why boilers have safety valves that are opened to avoid this. The expression "to let off some steam" comes from the fact that blowing some steam prevents explosions in the boiler. Nowadays, this idiom means "to relax".
Crocodile tears --------------- In the past, it was often believed that crocodiles shed tears that slid down into their mouths to moisten the food, making it easier to chew and swallow. Nowadays, this expression is used when someone pretends to cry to manipulate the situation. If someone sheds crocodile tears, it means they look sad or upset but in fact they are not.
To bark up the wrong tree ------------------------- This expression means to do something in a way that will not give you the information or result you want. It derives from the practice of using dogs for foxhunt. Dogs usually run after the fox until it is trapped in a tree, and they start barking up, in the direction of the fox. The idea of barking at a tree when the fox is actually not there, is regarded as something useless.