An idiom is a type of phrase or expression that has a meaning that can't be deciphered by defining the individual words.
dig in one's heels or dig one's heels in
to refuse to change one's course of action or opinion
Our boss dug in his heels and refused to give us time off.
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.