An idiom is defined as - a group of words in a fixed order that has a particular meaning that is different from the meanings of each word on its own.
know (something) inside out
to know something thoroughly
I know the history of our city inside out.
List of common idioms, their meanings and examples of their usage in sentences
An idiom is a group of words, or in other words, a phrase that has a meaning other than its literal meaning of the words in it.