Idiomatic expressions are a type of informal language that have a meaning different from the meaning of the words in expression. Here are some examples that recently came to mind.
Go Dutch or split the check means that each person pays an equal share of the bill [割り勘].
Pick up the tab means that the person who says it will pay the entire cost of the bill.
On the spur of the moment means on impulse; without planning.
Let the cat out of the bag means to reveal a secret carelessly or by mistake.
Jump (or leap) to conclusions means to make a hasty judgment before considering all the facts.
Beating a dead horse means to continue doing something that is a waste of time because the outcome is decided.
Out of the frying pan and into the fire is used to describe going from a bad situation to an even worse one.
See eye to eye means to be in agreement with someone.
Once in a blue moon means to do something very infrequently.
Cost an arm and a leg means very expensive.
A piece of cake is something that can be done very easily.
Feel under the weather means to not feel well.
Cut corners means to do something badly or cheaply.
Watch your tongue means you had better be careful about what you say.
Pot calling the kettle black is a situation where someone criticizes or blames another person of a fault he/she also has.
Not have a clue means to have no knowledge of something or be unable to remember something.
A lemon generally a car, but any other product, that has manufacturing defects or flaws.