The article Az észt szleng és kutatása discuss problems in defining the term slang. Slang is nowadays no longer taken to refer to language characteristic of the Underworld, but to what exactly it does refer to is, in the opinion of various eminent linguists, hard to pin down, and many of them have argued that no good single definition of the term exists. Attempts to shed light on the etymology of the term have also been unsuccessful. However it is possible to conclude that in the literature in English the word slang denotes the same linguistic phenomenon for which the terms argot and jargon are used in French and Russian sources, respectively. It is considered to be a colloquial type of language, which has a highly variable, transitory and informal vocabulary, and is characteristic of particular social groups, classes, professions or of circles of friends. According to an alternative definition, slang is a language which has two main characteristics - hypersynonymy and hyperpolysemy. Thus, the chief characteristic of slang is held to be total synonymality. Synonymality is a condition of all those lexemes for which it can be said that they do not consitute the primary definition for its designatum in any particular language. In the terms of this definition the vocabulary of slang consists of alternative linguistic expressions or units.
Among the various reasons for the use of slang we can note that it is conducive to secrecy (eavesdroppors may not figure out what the coded information means and that it is an economical and laconic means of communication. Also notable are the humour and novelty of slang, as well as the way in which it neatly allows the expression of highly judgemental evaluations of people and things. Slang, a universal phenomenon which can be found in every language, is a sign of vitality. Slang is produced only by a living language, i.e. slang gives evidence of vitality of its mother language. However, slang is a very ancient phenomenon, slang elements appear in Gilgamesh - the oldest surviving epic. The slang of Europe has a considerable history of its own, the first dictionaries of thieves' cant date back to the 15-th and l6-th centuries. The first attempt to collect thieves' cant in Estonia was made in 1914. A more systematic attempt to record slang was begun in 1926 at the University of Tartu - over the next eleven years, a Master's Thesis and various minor research projects were produced. At the same period articles dealing with slang were published in local periodicals. After the Second World War, however, research in to slang fell into disfavour. Nevertheless, collection of slang materials continued - the archives of the Estonian Native Language Society (Emakeele Selts) now contain about 50 000 cards of words and expressions, as well as several hundred pages of slang vocabulary. Nowadays the study of slang is undertaken at the level of higher education both in Tallinn as well as Tartu. A dictionary of the slang of Tallinn schoolchildren has been produced, and a Doctor's Thesis has also been written upon this theme.
Slang arises from the operation of general linguistic rules and procedures, for example, the items in the vocabulary of a standard language can take on wider meanings due to association - pillow (padi) can be used to refer to a sleepy person.
Foreign loans are another source of slang, perhaps especially so in the case of Estonian, where many foreign words seem so familiar (or so powerful) to the young that they use them in slang too, e.g. absull (meaning, 'absolutely'). The exact proportion of foreign items in contemporary Estonian slang varies according to the profession of the users of a particular form of slang. Musicians, and many others, have a large number of English loans, while in the prisons the Russian linguistic influence is more dominant.
Dialect words can also be used e.g. kabistama, to paw a girl.
Some archaisms are also preserved in slang, e.g. obrok meant historically 'ground rent', but it now marks a situation where a young boy gives some money or sweets to older boy(s).
Acronyms are used in slang, mainly for purposes of fun or language economy knr meaning 'rumour', formed from the initial letters of the phrase "keegi naine rääkis" ('some woman said').
Abbreviations are also popular, e.g. astro, for astronoomia ('astronomy').
Partial anagrams can make speech more colourful, e.g. poialpöiss, instead of pöialpoiss (in English, an equivalent would be Tum Thomb for Tom Thumb).
Metathesis, known in Estonian by the term sõnamäng-mõnasäng, is also popular, e.g. perutava hobuse norse ('the snort of the skittish horse') can become norutava hobuse perse ('the arse of the depressed horse').
Contraction or blends also occur, though not as frequently as in English, e.g. võrsikon for võõrsõnade leksikon ('a dictionary of foreign words').
Derivation is one very important way that slang can arise. Many suffixes, some of them normally nonproductive, are used to form Estonian slang terms, e.g. the suffixes kas, a, ar, er, ka, ku, s, u, ur, amongst others.