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.
Idiom:
sit in for (someone)
Meaning:
to take someone else's place in some activity
Example:
I asked my friend to sit in for me at my volunteer job at the community center.
List of common idioms, their meanings and examples of their usage in sentences
An idiom is defined as - a group of words whose meaning is different from the meanings of the individual words.