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:
tie down (something) or tie (something) down
Meaning:
to make something secure and unable to move by securing it with rope etc.
Example:
We tied down the pieces of wood on top of the car.
List of common idioms, their meanings and examples of their usage in sentences
An idiom is a group of words, or in other words, a phrase that has a meaning different from the literal meaning of the words in it.