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.
get out of bed on the wrong side or get out on the wrong side of bed
to be in a bad mood
I think that the girl got out of bed on the wrong side as she is not talking to anyone today.
List of common idioms, their meanings and examples of their usage in sentences
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.