IDIOM

IDIOM

An idiom (Latin: idiomī, "special property", from Greek: ἰδίωμα – idíōma, "special feature, special phrasing, a peculiarity", f. Greek: ἴδιος – ídios, "one's own") is a phrase or an expression that has a figurative, or sometimes literal, meaning. Categorized as formulaic language, an idiom's figurative meaning is different from the literal meaning.^[1] There are thousands of idioms, occurring frequently in all languages. It is estimated that there are at least twenty-five thousand idiomatic expressions in the English language. (https://en.wikipedia.org/wiki/Idiom)

Besides, according to oxford dictionary, idiom is a  group of words established by usage as having a meaning not deducible from those of the individual words. In addition, idiom is also mentioned as a characteristic mode of expression in music or art. In Oxford Dictionary, Idiom Originally, Late 16th century: from French idiome, or via late Latin from Greek idiōma ‘private property, peculiar phraseology’, from idiousthai ‘make one's own’, from idios ‘own, private’. (https://en.oxforddictionaries.com/definition/idiom)

 

We have known idiom base on Oxford Dictionary, so how about Cambridge ? according to Cambridge Dictionary, an idiom is a group of words in a fixed order that have a particular meaning that is different from the meanings of each word on its own, and another explanation Idiom is also the style of expression in writing, speech, or music that is typical of a particular period, person, or group.

(Definition of “idiom” from the Cambridge Advanced Learner’s Dictionary & Thesaurus © Cambridge University Press)

Examples of Idioms

 

There are tens of thousands of idioms used in the English language. As we can probably understand, idioms are difficult for people whose first language is not English. Actually, even native English speakers in the Motherland of England have a hard time with American idioms. Americans know them because they're common expressions. We're used to hearing them in our homes, at school, and in our favorite television programs.

Sometimes American idioms make their way around the world, and sometimes an expression doesn't even make it out of a particular state. Here a few of the most commonly used idioms in the United States and their likely origins. (Note that these origins may be in dispute.) 

'As happy as a clam'

Mary was happy as a clam on the day of her wedding.

Meaning: Pleased, very content

Origin: So why in the world would a clam ever be happy? They're shellfish, they don't have feelings. The expression most likely began because when a clam's shell is opened up, it does look like the clam is smiling. 

'Pass the buck'

It is Tim's responsibility to mow the lawn. He shouldn't pass the buck to his younger brother.

Meaning: To dodge one's responsibilities by giving them to someone else

Origin: The game of poker became extremely popular in the 1800s in the United States. Of course, many of the cowboys and bar patrons who played the game were degenerate gambler types who could not be trusted. So, a system was instilled that on each new game a different person would deal out the cards and they would be given some kind of marker to indicate that it was their deal. Often times a knife was used as a marker. The handle of these knives were typically made out of a buck's horn. In time, the marker just became known as a buck.  (study.com)

to give more explanation about idiom, i suggest you to read this thesis by Julia Holmqvist 

http://www.diva-portal.org/smash/get/diva2:791008/FULLTEXT01.pdf