Slang fills a necessary niche in all languages, occupying a middle ground between the standard and informal words accepted by the general public and the special words and expressions known only to comparatively small social subgroups. It can serve as a bridge or a barrier, either helping both old and new words that have been used as "insiders' " terms by a specific group of people to enter the language of the general public or, on the other hand, preventing them from doing so. Thus, for many words, slang is a testing ground that finally proves them to be generally useful, appealing, and acceptable enough to become standard or informal. For many other words, slang is a testing ground that shows them to be too restricted in use, not as appealing as standard synonyms, or unnecessary, frivolous, faddish, or unacceptable for standard or informal speech. For still a third group of words and expressions, slang becomes not a final testing ground that either accepts or rejects them for general use but becomes a vast limbo, a permanent holding ground, an area of speech that a word never leaves. Thus, during various times in history, American slang has provided cowboy, blizzard, okay, racketeer, phone, gas, and movie for standard or informal speech. It has tried and finally rejected conbobberation (disturbance), krib (room or apartment), lucifer (match), tomato (girl), and fab (fabulous) from standard or informal speech. It has held other words such as bones (dice), used since the 14th century, and beat it (go away), used since the 16th century, in a permanent grasp, neither passing them on to standard or informal speech nor rejecting them from popular, long-term use.

Slang words cannot be distinguished from other words by sound or meaning. Indeed, all slang words were once cant, jargon, argot, dialect, nonstandard, or taboo. For example, the American slang to neck (to kiss and caress) was originally student cant; flattop (an aircraft carrier) was originally navy jargon; and pineapple (a bomb or hand grenade) was originally criminal argot. Such words did not, of course, change their sound or meaning when they became slang. Many slang words, such as blizzard, mob, movie, phone, gas, and others, have become informal or standard and, of course, did not change in sound or meaning when they did so. In fact, most slang words are homonyms of standard words, spelled and pronounced just like their standard counterparts, as for example (American slang), cabbage (money), cool (relaxed), and pot (marijuana). Of course, the words cabbage, cool, and pot sound alike in their ordinary standard use and in their slang use. Each word sounds just as appealing or unappealing, dull or colourful in its standard as in its slang use. Also, the meanings of cabbage and money, cool and relaxed, pot and marijuana are the same, so it cannot be said that the connotations of slang words are any more colourful or racy than the meanings of standard words.

All languages, countries, and periods of history have slang. This is true because they all have had words with varying degrees of social acceptance and popularity.

All segments of society use some slang, including the most educated, cultivated speakers and writers. In fact, this is part of the definition of slang. For example, George Washington used redcoat (British soldier); Winston Churchill used booze (liquor); and Lyndon B. Johnson used cool it (calm down, shut up).

The same linguistic processes are used to create and popularize slang as are used to create and popularize all other words. That is, all words are created and popularized in the same general ways; they are labeled slang only according to their current social acceptance, long after creation and popularization.

Slang is not the language of the underworld, nor does most of it necessarily come from the underworld. The main sources of slang change from period to period. Thus, in one period of American slang, frontiersmen, cowboys, hunters, and trappers may have been the main source; during some parts of the 1920s and '30s the speech of baseball players and criminals may have been the main source; at other times, the vocabulary of jazz musicians, soldiers, or college students may have been the main source.

To fully understand slang, one must remember that a word's use, popularity, and acceptability can change. Words can change in social level, moving in any direction. Thus, some standard words of William Shakespeare's day are found only in certain modern-day British dialects or in the dialect of the southern United States. Words that are taboo in one era (e.g.,