Idioms can be a powerful linguistic tool only when they are used in the right way in the right places.
Idiom:
behind (someone's) back
Meaning:
without someone`s knowledge, secretly
Example:
The man is very angry because his friend borrowed his car behind his back.
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.