An idiom is a type of phrase or expression that has a meaning that can't be deciphered by defining the individual words.
Idiom:
sell out (someone or something) or sell (someone or something) out
Meaning:
to be disloyal, to betray someone or something
Example:
The man does not want to sell out his moral values.
List of common idioms, their meanings and examples of their usage in sentences
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.