The concept of anti-languages was first defined and studied by the linguist Michael Halliday, who used the term to describe the lingua franca of an anti-society. He defined an anti-language as a language created and used by an anti-society. An anti-society is a small, separate community intentionally created within a larger society as an alternative to or resistance of it. For example, Adam Podgorecki studied one anti-society composed of Polish prisoners; Bhaktiprasad Mallik of Sanskrit College studied another composed of criminals in Calcutta.
Anti-languages are developed by these societies as a means to prevent outsiders from understanding their communication, and as a manner of establishing a subculture that meets the needs of their alternative social structure. Anti-languages differ from slang and jargon in that they are used solely among ostracized or rebellious social groups including prisoners, criminals, homosexuals, and teenagers. Anti-languages use the same basic vocabulary and grammar as their native language in an unorthodox fashion. For example, anti-languages borrow words from other languages, create unconventional compounds, or utilize new suffixes for existing words. Anti-languages may also change words using metathesis, back formation (e.g. apple to elppa), or by substituting their consonants. Therefore, anti-languages are distinct and unique, and are not simply dialects of existing languages.
In his essay "Anti-Language", Halliday synthesized the research of Thomas Harman, Adam Podgórecki, and Bhaktiprasad Mallik to explore anti-languages and the connection between verbal communication and the maintenance of social structure. For this reason, the study of anti-languages is both a study of sociology and linguistics. Halliday's findings can be compiled as a list of nine criteria that a language must meet to be considered an anti-language:
1. An anti-society is a society which is set up within another society as a conscious alternative to it.
2. Like the early records of the languages of exotic cultures, the information usually comes to us in the form of word lists.
3. The simplest form taken by an anti-language is that of new words for old: it is a language relexicalised.
4. The principle is that of same grammar, different vocabulary.
5. Effective communication depends on exchanging meanings which are inaccessible to the layperson.
6. The anti-language is not just an optional extra, it is the fundamental element in the existence of the “second life” phenomenon.
7. The most important vehicle of reality-maintenance is conversation. All who employ this same form of communication are reality-maintaining others.
8. The anti-language is a vehicle of resocialisation.
9. There is continuity between language and anti-language.
Examples of anti-languages include cockney rhyming slang, CB slang, Verlan, the grypsera of Polish prisons, thieves' cant, Polari, and possibly Palawa kani and Bangime.