An idiom is a type of phrase or expression that has a meaning that can't be deciphered by defining the individual words.
kiss of death
an act that puts an end to someone or something
It was the kiss of death for the conversation with the teacher when the student learned that the teacher knew her father.
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.