One use of slang is to circumvent social taboos, as mainstream language tends to shy away from evoking certain realities. For this reason, slang vocabularies are particularly rich in certain domains, such as sexuality, violence, crime and drugs. Alternatively, slang can grow out of mere familiarity with the things described. Among wine connoisseurs, Cabernet Sauvignon might be known as "Cab Sav", Chardonnay as "Chard" and so on;[2] this means that naming the different wines expends less superfluous effort. It also serves as a shared code among connoisseurs. Even within a single language community, slang tends to vary widely across social, ethnic, economic, and geographic strata. Slang sometimes grows more and more common until it becomes the dominant way of saying something, at which time it is regarded as mainstream, acceptable language (e.g. the Spanish word caballo), while at other times it may fall into disuse. Numerous slang terms pass into informal mainstream speech, and sometimes into formal speech, though this may involve a change in meaning or usage. Slang very often involves the creation of novel meanings for existing words. It is very common for such novel meanings to diverge significantly from the standard meaning. Thus, "cool" and "hot" can both mean "very good, impressive or good looking." Slang terms are often only known within an ingroup. For example, Leet Speak (Leet or "1337") is popular among online video gamers (although it is slowly being picked up by gaming in general). Text language is also a different form of slang used among people to shorten words when typing. http://en.wikipedia.org/wiki/Slang Vernacular vocabulary not generally acceptable in formal usage. It is notable for its liveliness, humor, emphasis, brevity, novelty, and exaggeration. Most slang is faddish and ephemeral, but some words are retained for long periods and eventually become part of the standard language (e.g., phony, blizzard, movie). On the scale used to indicate a word’s status in the language, slang ranks third behind standard and colloquial (or informal) and before cant. Slang often conveys an acerbic, even offensive, no-nonsense attitude and lends itself to poking fun at pretentiousness. Frequently grotesque and fantastic, it is usually spoken with intent to produce a startling or original effect. It is especially well developed in the speaking vocabularies of cultured, sophisticated, linguistically rich languages. Characteristically individual, slang often incorporates elements of the jargons of special-interest groups (e.g., professional, sport, regional, criminal, and drug subcultures). Slang words often come from foreign languages or are of a regional nature. Slang is very old, and the reasons for its development have been much investigated. The following is a small sample of American slang descriptive of a broad range of subjects: of madness—loony, nuts, psycho; of crime—heist, gat, hit, heat, grifter; of women—babe, chick, squeeze, skirt; of men—dude, hombre, hunk; of drunkenness—sloshed, plastered, stewed, looped, trashed, smashed; of drugs—horse, high, stoned, tripping; of caressing—neck, fool around, make out; of states of mind—uptight, wired, mellow, laid back; the verb to go—scram, split, scoot, tip; miscellaneous phrases—you push his buttons, get it together, chill, she does her number, he does his thing, what’s her story, I’m not into that. http://www.bartleby.com/65/sl/slang.html The known history of European slang begins (leaving out of account the meagre references in German documents hereaf ter to be mentioned) with the "Ballades" of Francois Villon in the 15th century. The French argot of these compositions contains much that is still obscure, but the origin of some of its words is evident enough. Facetious expressions relating to the destined end of the malefactor are prominent. Paroir and montjoye (for which later the less ironical monte a regret was substituted) are nicknames for the scaffold. Acollez, hanged, corresponds to the English "scragged"; the synonymous grup seems to be an onomatopoeic formation suggestive of choking. There are some derivatives formed with the suffix art: riflart is a police-officer, abroieart fog. A few words from foreign languages occur: audinos, prayer, is the Latin audi nos of the litanies; arton, bread, is obviously Greek, and its appearance in the 15th century is somewhat hard to account for. Moller, to eat, may perhaps be the Latin molere to grind. Anse, the ear, is no doubt the Latin ansa, handle. In the 15th century and later the ranks of vagabondage were often recruited from the class of poor students, so that the presence of some words of learned origin in the vocabulary of the vagrant and criminal classes is not surprising. Among the prominent features of later French slang may be noted the use of the suffix mare to form derivatives such as perruquemare, a wig-maker, and the practice of rendering conversation unintelligible to outsiders by tacking on some unmeaning ending to every word. http://www.1911encyclopedia.org/Slang That’s true of all the sources I’ve consulted, as well. Nobody seems to know where the word comes from (though everyone is sure it is not a compound of any word with language). It appeared in the eighteenth century, itself as what we would now call a slang term, with various meanings, of which our modern sense is the only one that has survived. Some have tried to trace it back to a Norwegian form sleng-, implying offensive language (found only in compounds) but others deny that connection on the grounds that the dates don’t support a link. There is a modern facetious formation slanguage, but that’s an obvious blend of slang and language. http://www.worldwidewords.org/qa/qa-sla1.htm Slang tends to originate in subcultures within a society. Occupational groups (for example, loggers, police, medical professionals, and computer specialists) are prominent originators of both jargon and slang; other groups creating slang include the armed forces, teenagers, racial minorities, ghetto residents, labor unions, citizens-band radio broadcasters, sports groups, drug addicts, criminals, and even religious denominations (Episcopalians, for example, produced spike, a High Church Anglican). Slang expressions often embody attitudes and values of group members. They may thus contribute to a sense of group identity and may convey to the listener information about the speaker's background. Before an apt expression becomes slang, however, it must be widely adopted by members of the subculture. At this point slang and jargon overlap greatly. If the subculture has enough contact with the mainstream culture, its figures of speech become slang expressions known to the whole society. http://encarta.msn.com/encyclopedia_761563998/Slang.html

That depends specifically what slang you're talking about. Some could be from popular plays, operas, books, etc.