Idioms are phrases which cannot be understood simply by looking at the meaning of the individual words in the phrase.
Idiom:
knuckle under
Meaning:
to yield, to submit
Example:
The union finally knuckled under from the pressure and ended the strike.
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.