Idioms...every language has them but to language learners they pose quite the obstacle. They are phrases that you just have to learn and memorize - guessing the meaning might lead to some potentially awkward 'lost in translation' situations. So with that in mind, here are some English idioms, their meanings and origins.
1: Raining cats and dogs
Meaning: It is no secret that us Brits are slightly obsessed with the weather, so of course we have to include a weather related idiom. "Its raining cats and dogs" means that it is raining particularly heavily.
Origin: Speculation to the origins of this phrase range from medieval superstition to Norse mythology but another slightly disturbing interpretation is that it is a reference to dead animals being washed through the streets by floods! :o
2: Mad as a hatter
Meaning: This phrase refers to someone who is completely crazy. "He's a mad as a hatter". A similar expression is 'Mad as a march hare'.
Origin: The hatter here refers to Lewis Carroll's mad hatter character in Alice in Wonderland. However, the origins of this expression lie in the effects of mercury poisoning that 18th and 19th century hat makers suffered from. Mad as a march hare refers to the behaviour of hares during the mating season.
3: In Stitches
Meaning: If you are 'in stitches' this means that you are laughing so hard, your sides hurt.
Origin: It presumably is a comparison of the pain of laughter to that with a prick of a needle. The first recorded use of the phrase is of course by none other than Shakespeare in Twelfth Night.
4: Driving me up the wall
Meaning: This is used when something (or someone) is causing extreme exasperation or annoyance. A similar expression is 'your driving me round the bend'. A phrase many parents have used.
Origins: While its first use is unknown, it refers to someone desperately trying to escape by climbing the walls.
5: Head in the clouds
Meaning: Used to describe someone who is not realistic, it suggests they are not grounded in reality and prone to flights of fancy. For example, 'He's not right for this job, he has his head in the clouds'.
Origin: It has been used since the 1600's although the origins are rather unclear apart from the obvious image of someone who's a fantasist - having one's head in a cloud is clearly impossible.
More idioms and their origins here