Differences and Usage in American English and British English
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.)