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.
put one's thinking cap on or put on one's thinking cap
to start thinking in a serious manner
I put my thinking cap on and tried to think of a name for the student newspaper.
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.