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.
Idiom:
get (something) through (someone's) thick skull
Meaning:
to get someone to understand something
Example:
I could not get it through my friend's thick skull that I could not go on a holiday with him.
List of common idioms, their meanings and examples of their usage in sentences
Idioms are phrases which cannot be understood simply by looking at the meaning of the individual words in the phrase.