A group of words which have a different meaning when used together from the one they would have if you took the meaning of each word separately.
lay down the law
to tell someone what to do by using your power or influence
The new manager plans to lay down the law to the workers regarding long lunch breaks.
List of common idioms, their meanings and examples of their usage in sentences
Idioms are phrases which cannot be understood simply by looking at the meaning of the individual words in the phrase.