The Slang reference article from the Simple Wikipedia on 24-Jul-2004
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Slang

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Slang is jargon used by the general public. Usually each generation or ethnic group has its own - older people often have trouble understanding the slang of younger people. By contrast, younger people will very often understand, but find silly or old-fashioned, the slang of older people.

Over time, language tends to get more complex, since new words enter much faster than old words leave. Over time, slang almost always becomes part of the language and approved for use by all. It has also happened that words, like the Anglo-Saxon words piss, shit and fuck, are defined as being rude by powerful people who do not use them. To this day, these words remain as English slang, after almost a thousand years since these words were formally replaced by the Norman words urinate, defecate and copulate - which polite people were supposed to use. This was in part a way of making poor people (who spoke Anglo-Saxon) all appear to be rude while rich people (who spoke Norman) appeared to be polite - one way that etiquette can develop, and reinforce power structure. Racism is one motive for defining one group's language as slang and another as correct.

An idiom can be slang, but it can also be a metaphor that becomes part of the culture.