How do secret languages emerge?

The very concept of ‘secret’ languages appears as if it is taken out of a crime novel. We may think of military secret codes, jargons by criminal inmates, or suburban youth slang. However, are not all languages (except for, let us say, standard English) in some sense ‘secret’, as long as they are spoken by a close group of people and unintelligible to outsiders? This is true in many cases: for instance, minority languages, immigrant languages, local languages or dialects, youth jargons, or ethnolects – these represent communication systems that are restricted to a closed group of speakers and not shared by outsiders. So what makes a language a ‘secret’ language? The answer is complex.
A secret language is no one’s mother tongue – this is probably the most important distinction from a ‘normal’ language. Rather, secret languages represent traditionally a jargon that was transferred from father to son, together with an occupation or a life-style, the purpose of which was not just to keep outsiders, but also members of the own family, outside. Secret languages, connected to various occupations, are found in Europe as well as in Africa and South America. They are very often the idiom of occupations with a distinct social function, most typical occupations that are excerpted within the society but which have with a special, often low, status. In Europe, pedlars, dealers, chimney sweepers or circus people, but also various types of low-status occupations, such as the executioner's henchman or skinners, used to have their own secret languages. In Africa, to mention an example, we have documented secret languages among healers, skinners, and sandal flickers. 
Linguistically, secret languages do not possess their own grammar, like ‘normal’ languages do. Their grammatical system relies on the grammar system of another language, most normally the majority language of the country where they occur. The grammar is often simplified and syntactic patterns are replaced by pidgin-like structures. A frequently occurring phenomenon is to borrow the ‘appearance’ of a language, by means of stress patterns, prosody, dialectal variation and gestures, but to switch all content words, sometimes the entire lexicon. The lexicon is either taken more or less completely from another language, or it is an ad hoc-conglomerate of words from various adjacent source languages. Very often, secret languages ‘distort’ their words by various complex patterns of morphological transformation; for instance, they truncate words and add heavy suffixes, they reverse syllables or letters, or they add epenthetic vowels within words. The result is a language that ‘melts in’ – from distance they appear as if they are a native or indigenous idiom, but not one single word is understandable to outsiders.

On Scandinavian soil, there are several traditional secret languages. One is the pedlars language, which in fact is two, one in the isolated county of Dalecarlia, gråmål ‘grey language’ or monsing, the main pedlars’ secret language, which during the 20th ct. transformed into a prisoners’ language. The vocabulary of monsing is based on multiple languages. Many words are borrowed from Scandoromani, the language of the indigenous Swedish Romani speakers, other words are from Low German, Rotwelsch, the Medieval secret jargon of European outsiders, from Finnish, Russian, as well as from Swedish. Swedish words are totally changed by linguistic distortion. Sources of monsing go back to the 17th ct. and they give us a glimpse of the type of communication that monsing speakers had. Besides communication related to their occupation, much of the content is rude, such as talk is about the farmers (who are supposed to be stupid) and in particular their wives and daughters (who are target of their sexual interest).
Even though there are no ‘real’ speakers of these languages in Sweden anymore, monsing is still, together with Scandoromani and knoparmoj, the secret language of chimney-sweepers, a very important source for words in the Scandinavian vernacular languages.

Reference
Carling, Gerd, Lenny Lindell & Gilbert Ambrazaitis (2014) Scandoromani. Remnants of a Mixed Language. Boston: Brill

Samples of the Swedish secret language Monsing (from 18th and 19th ct. sources). Most of them have found their way into Swedish slang.