An idiom is a word or phrase which means something different from what it says – it is usually a metaphor. Idioms are common phrases or terms whose meaning is not real, but can be understood by their popular use.
Because idioms can mean something different from what the words mean, it is difficult for someone not very good at speaking the language to use them properly. Some idioms are only used by some groups of people or at certain times.
The idiom shape up or ship out, which is like saying improve your behavior or leave if you don’t, might be said by an employer or supervisor to an employee, but not to other people.
Idioms are not the same thing as slang. Idioms are made of normal words that have a special meaning known by almost everyone. Slang is usually special words that are known only by a particular group.
To learn a language a person needs to learn the words in that language, and how and when to use them. But people also need to learn idioms separately because certain words together or at certain times can have different meanings. In order to understand an idiom, one sometimes needs to know the culture the idiom comes from.
To know the history of an idiom can be useful and interesting, but is not necessary to be able to use the idiom properly. For example most native British English speakers know that “No room to swing a cat” means “there was not a lot of space” and can use the idiom properly, but few know it is because 200 years ago sailors were punished by being whipped with a whip called a “cat o’nine tails”. A big space was cleared on the ship so that the person doing the whipping had a lot of room to swing the cat.
A better understanding of an idiom is that it is a phrase whose meaning cannot be understood from the dictionary definitions of each word taken separately.
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