Psychologically, most good slang harks back to the stage in human culture when animism was a worldwide religion. At that time, it was believed that all objects had two aspects, one external and objective that could be perceived by the senses, the other imperceptible (except to gifted individuals) but identical with what we today would call the "real" object. Human survival depended upon the manipulation of all "real" aspects of life--hunting, reproduction, warfare, weapons, design of habitations, nature of clothing or decoration, etc.--through control or influence upon the animus, or imperceptible phase of reality. This influence was exerted through many aspects of sympathetic magic, one of the most potent being the use of language. Words, therefore, had great power, because they evoked the things to which they referred.
Civilized cultures and their languages retain many remnants of animism, largely on the unconscious level. In Western languages, the metaphor owes its power to echoes of sympathetic magic, and slang utilizes certain attributes of the metaphor to evoke images too close for comfort to "reality." For example, to refer to a woman as a "broad" is automatically to increase her girth in an area in which she may fancy herself as being thin. Her reaction may, thus, be one of anger and resentment, if she happens to live in a society in which slim hips are considered essential to feminine beauty. Slang, then, owes much of its power to shock to the superimposition of images that are incongruous with images (or values) of others, usually members of the dominant culture. Slang is most popular when its imagery develops incongruity bordering on social satire. Every slang word, however, has its own history and reasons for popularity. When conditions change, the term may change in meaning, be adopted into the standard language, or continue to be used as slang within certain enclaves of the population. Nothing is flatter than dead slang. In 1910, for instance, "Oh you kid" and "23-skiddoo" were quite stylish phrases in the U.S. but they have gone with the hobble skirt. Children, however, unaware of anachronisms, often revive old slang under a barrage of older movies rerun on television.
Some slang becomes respectable when it loses its edge; "spunk," "fizzle," "spent," "hit the spot," "jazz," "funky," and "p.o.'d," once thought to be too indecent for feminine ears, are now family words. Other slang survives for centuries, like "bones"
While many slang words introduce new concepts, some of the most effective slang provides new expressions--fresh, satirical, shocking--for established concepts, often very respectable ones. Sound is sometimes used as a basis for this type of slang, as, for example, in various phonetic distortions (e.g., pig Latin terms). It is also used in rhyming slang, which employs a fortunate combination of both sound and imagery. Thus, gloves are "turtledoves" (the gloved hands suggesting a pair of billing doves), a girl is a "twist and twirl" (the movement suggesting a girl walking), and an insulting imitation of flatus, produced by blowing air between the tip of the protruded tongue and the upper lip, is the "raspberry," cut back from "raspberry tart." Most slang, however, depends upon incongruity of imagery, conveyed by the lively connotations of a novel term applied to an established concept. Slang is not all of equal quality, a considerable body of it reflecting a simple need to find new terms for common ones, such as the hands, feet, head, and other parts of the body. Food, drink, and sex also involve extensive slang vocabulary. Strained or synthetically invented slang lacks verve, as can be seen in the desperate efforts of some sportswriters to avoid mentioning the word baseball--e.g., a batter does not hit a baseball but rather "swats the horsehide," "plasters the pill," "hefts the old apple over the fence," and so on.
The most effective slang operates on a more sophisticated level and often tells something about the thing named, the person using the term, and the social matrix against which it is used. Pungency may increase when full understanding of the term depends on a little inside information or knowledge of a term already in use, often on the slang side itself. For example, the term Vatican roulette (for the rhythm system of birth control) would have little impact if the expression Russian roulette were not already in wide usage.