An idiom is a figure of speech that is particular to a given language, may lack standard grammatical structure, and may possess a figurative meaning. Because of their often unique constructions and non-literal meanings, many idioms cannot be translated directly into other languages.

Idiom derives from the Greek word idioma, from idiousthai (“to appropriate”). 

Many idioms that are popular today originated from literature. For example, the expression “wear my heart on my sleeve” first appeared in William Shakespeare’s Othello, in a line spoken by Iago in act 1, scene 1:

For when my outward action doth demonstrate

The native act and figure of my heart

In complement extern, 'tis not long after

But I will wear my heart upon my sleeve

For daws to peck at. I am not what I am.

Iago uses this idiom to tell Roderigo that revealing one’s true motives makes one vulnerable. By exposing his heart, Iago would be putting it in danger of being pecked by “daws,” or jackdaws, an extension of the figure that makes vivid the dangers of such vulnerability. The phrase has become common in English usage to describe the act of being vulnerable, but translated literally into another language it would not necessarily retain that meaning, even if the image itself was understood. 

see: colloquialism, figure of speech