When the čaršijas* [*To make reading easier the ‘bchs’ version of the word (čaršija) is used in the texts on Bosnia Herzegovina and Serbia; in those on Albania, the Albanian spelling (çarshija); whereas for the bazaars in Kosovo and Macedonia both wordings are used indifferently] were the pulsating heart of Balkan towns, many languages mingled and crossbred there. Similar languages belonging to the southern Slav family, but cousins of Greek, Albanian and Rumanian, part of the Balkan brotherhood formed over the centuries with population movement and the outside influence of German, Italian, Venetian and Hungarian. In the çarshijas everyone was multilingual with each person being able to speak to his neighbour in that person’s own language.
Along with the natural languages, defying the principles of historic linguistics, invented languages survived and even developed over the centuries. These were actual codes which the people of the čaršijas used to communicate. Secret languages with precise grammar rules and vocabulary were conceived in such a way as to confuse any outsider who tried to guess the meaning.
The secret languages existed for many centuries, probably since the birth of the çarshijas, though when they began cannot be precisely dated though they have been found in many texts from 1500. Some words and expressions survived in urban jargon in several towns but few people have studied them and with the emptying of the čaršijas they have become extinct. […]
According to the theory of Milenko Filipović, supported by the Kosovan Kadri Halimi, the secret languages respected the grammar rules of the local majority language, for example Serbo-Croat in Sarajevo, inserting words which were invented also from a morphological point of view. “Often the words were metaphors which then took on another meaning” states Filipović in a document of 1930. For instance gledač meant window, derived from the verb gledati – to look; pevac – pope [presumably ‘Orthodox priest’ — LH], from the verb pevati – to sing; ušačka – door, from the verb ući – to enter.
Another system was the so-called ters. This consisted in using the opposite of the root of the word. Somewhat similar was the šatrovački system in which the order of the syllables in a word was changed. An example of the latter can be found in the Albanian saying shatra-patra which probably derives from the Romani shatra, here indicating a Rom word inserted into other languages.
There are a lot more examples at the link; thanks, Michael!