If you look up the word idiom in Webster, you will be given the following definition: Idiom is an expression whose meaning is not predictable from the usual meanings of its constituent element as kick the bucket, hang one's head etc., or from the general grammatical rules of language, as the table round for the round table, and which is not a constituent of a larger expression of like characteristics. This definition seems a bit dry and doesn't really tell anything about the function of idioms in English language.
English is a language particularly rich in idioms - those modes of expression peculiar to a language (or dialect) which frequently defy logical and grammatical rules. Without idioms English would lose much of its variety and humor both in speech an writing.
The background and etymological origins of most idioms is at best obscure. This is the reason why a study of differences between the idioms of American and British English is somewhat difficult. But it also makes the cases, where background, etymology and history are known, even more interesting. Some idioms of the "worldwide English" have first been seen in the works of writers like Shakespeare, Sir Walter Scott, Lewis Carroll or even in the paperbacks of contemporary novelists. An example of Shakespearian quotation can be found in the following sentence:"As a social worker, you certainly see the seamy side of life." Biblical references are also the source of many idioms. Sports terms, technical terms, legal terms, military slang and even nautical expressions have found their way to the everyday use of English language. Following are some examples of these, some used in either American or British English and some used in both:
"Having won the first two Tests, Australia is now almost certain to retain the Ashes." (Ashes is a British English idiom that is nowadays a well-established cricket term.)
"In his case the exception proves the rule." (A legal maxim -- in full:"the exception proves the rule in cases not excepted". Widely used in both AmE and BrE.)
"To have the edge on/over someone." (This is originally American English idiom, now established in almost every other form of English, including BrE.)
"A happy hunting ground." (Place where one often goes to obtain something or to make money. Originally American English idiom from the Red Indians' Paradise.)
In the old days English idioms rarely originated from any other form of English than British English. (French was also a popular source of idioms.) Nowadays American English is in this position. It is hard to find an AmE idiom that has not established itself in "worldwide English" (usually BrE). This is not the case with British English idioms which are not as widespread. It has to be remembered that it is hard to say which idioms are actively used in English and which are dying out or have already died. Idioms are constantly dying and new-ones are born.
Some idioms may have gone through radical changes in meaning. The phrase - There is no love lost between them - nowadays means that some people dislike one another. Originally, when there was only the British English form, it meant exactly the opposite. The shift in meaning is yet unexplained. All dialects of English have different sets of idioms and situations where a given idiom can be used. American English and British English may not, in this respect, be the best possible pair to compare because they both have been developing into the same direction, at least where written language is concerned, since the Second World War. The reason that there is so much American influence in British English is the result of the following:
Magnitude of publishing industry in the U.S.
Magnitude of mass media influence on a worldwide scale
Appeal of American popular culture on language and habits worldwide
International political and economic position of the U.S.
All these facts lead to the conclusion that new idioms usually originate in the U.S. and then become popular in so-called "worldwide English". This new situation is completely different from the birth of American English as a "variant" of British English. When America was still under the rule of the Crown, most idioms originated from British English sources. Of course there were American English expressions and idioms too, before American English could be defined as dialect of English. Some examples of these early American English idioms follow:
"To bark up the wrong tree." (Originally from raccoon-hunting in which dogs were used to locate raccoons up in trees.)
"Paddle one's own canoe." (This is an American English idiom of the late 18th Century and early 19th Century.)
Some of these early American idioms and expressions were derived from the speech of the American natives like the phrase that "someone speaks with a forked tongue" and the "happy hunting ground" above. These idioms have filtered to British English through centuries through books, newspapers and most recently through powerful mediums like radio, TV and movies.
Where was the turning point? When did American culture take the leading role and start shaping the English language and especially idiomatic expressions? There is a lot of argument on this subject. Most claim that the real turning point was the Second World War. This could be the case. During the War English-speaking nations were united against a common enemy and the U.S. took the leading role. In these few years and a decade after the War American popular culture first established itself in British English. Again new idioms were created and old ones faded away. The Second World War was the turning point in many areas in life. This may also be the case in the development of the English language.
In the old days the written language (novels, poems, plays and the Bible) was the source from which idioms were extracted. This was the case up until WWII. After the war new mediums had established themselves in English-speaking society, there was a channel for the American way of life and the popular culture of the U.S. TV, movies and nowadays the interactive medium have changed the English language more to the American English direction. Some people in the Europe speak the Mid-Atlantic English, halfway from the British English to American English.
The influence of American English can even be seen in other European languages. In Finland, we are adopting and translating AmE proverbs, idioms and expressions. It can be said that the spoken language has taken the leading role over the written and the only reason for this is TV and radio. Most proverbs and idioms that have been adopted to British English from American English are of spoken origin. This is a definite shift from the days before WWII. What will this development do to the English language? Will it decrease its value? This could be argued, but the answer would still be no. Languages develop and change. So is the case with English language and idioms.
How then does American English differ from British English in the use of idioms? There are no radical differences in actual use. The main differences are in the situations where idiomatic expressions are used. There have been many studies recently on this subject. American English adopts and creates new idioms at a much faster rate compared to British English. Also the idioms of AmE origin tend to spread faster and further. After it has first been established in the U.S., an American idiom may soon be found in other "variants" and dialects of English. Nowadays new British idioms tend to stay on the British Isles and are rarely encountered in the U.S. British idioms are actually more familiar to other Europeans or to the people of the British Commonwealth than to Americans, even though the language is same. The reason for all these facts is that Britain is not the world power it used to be and it must be said that the U.S. has taken the role of the leading nation in the development of language, media and popular culture. Britain just doesn't have the magnitude of media influence that the United States controls.
The future of idiomatic expressions in the English language seems certain. They are more and more based on American English. This development will continue through new mediums like the Internet and interactive mediums. It is hard to say what this will do to idioms and what kind of new idioms are created. This will be an interesting development to follow, and by no means does it lessen the humor, variety and color of English language.