An idiom is a type of phrase or expression that has a meaning that can't be deciphered by defining the individual words.
Idiom:
knock oneself out (doing something)
Meaning:
to make a great effort doing something
Example:
We knocked ourselves out trying to make the party successful.
List of common idioms, their meanings and examples of their usage in sentences
An expression in the usage of a language that is peculiar to itself either in having a meaning that cannot be derived from the conjoined meanings of its elements or in its grammatically atypical use of words.