An idiom is a type of phrase or expression that has a meaning that can't be deciphered by defining the individual words.
Idiom:
build up (someone or something) or build (someone or something) up
Meaning:
to make someone or something bigger or stronger, to promote someone or something
Example:
The woman is always trying to build up her boss.
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.