Warning: Using this term could get you punched in the teeth at a linguistics convention.
It was coined by M. A. K. Halliday, and is itself quite the loaded term, currently deemed be many in the field to be a bit too us-and-them for language as it exists in the world. It is however a useful tool for describing an aspect of the structure of language as it is broken down into dialect, sociolect, and idiolect, words known to such paragons of knowledge as liveforever and Pseudo_Intellectual, but which Webby likely never saw coming.
Antilanguage--you'll often see that in scare quotations--is most closely a subset of sociolect, and is used to define or describe, unsurprisingly, a 'language' in antagonistic opposition to the standard. Consequently, it is linked to antagonistic, peripheral, and/or underground subcultures that are often seen as undesirable by the majority or ruling minority. The languages are often secret, highly coded, and all but incomprehensible to outsiders, resulting in the creation and reinforcement of solidarity amongst their users. That solidarity, however, frequently defines the borders of a linguistic community, and users of the language who do not or cannot also employ the "standard" run the risk of being stereotyped and permanently associated with the "undesirable" (Roger Fowler uses the word "deviant," note that connotation) elements of their particular subculture.
Essentialy, slang plus plus. It takes more than a few altered word forms or stand-ins to qualify as antilanguage.
If that word hasn't been around long, the phenomenon certainly has, already in good, recordable form by the 16th Century. Elizabethan thieves and vagabonds used what Halliday describes as a "pelting speech" to identify each other and cloak their activities. The underworld of Calcutta has a language all its own, and what is transliterated as grypserka has penetrated and flourished throughout the entire inmate population of the Polish prison system.
An excellent literary example of an antilanguage is the 'Nadsat' speech of Alex and Droogs in A Clockwork Orange, grounded heavily in Russian and in some editions of the novel explained by an appendix or glossary. But it isn't all to do with nicking pretty polly from the trees and cheezing it when the millicents get wise.
Halliday points out that antilanguage is also a medium of great creative opportunity, as it is based on the standard language, but significantly manipulated. Phonetic mutation and metaphor give rise to a new identifying vocabulary that lends itself to word-play and verse. Fowler identifies antilanguage in rap music, Cockney rhyming slang, Rastafarian poetry, and even the highly specific jargon of truckers. For e2 writeups, check out palare.