From the journal Πεταχτό Κόρτε (Fleeting Flirt), “one of the risqué magazines of the time, with half-naked women drawn on the front cover, cartoons with innuendo-laced captions showing ladies in negligées, poems and witticisms full of double entendres”. Source.
Further to the French Verlan, and the secret language of blind musicians in China, the work of Elias Petropoulos (see under Rebetika) led me to Kaliarda, the cant of underworld homosexuals in Athens. Nick Nicholas has written a whole series of twenty-four erudite articles online, starting here.
The speakers of Kaliarda were a cohesive social group, who associated with each other, had their own tavernas and beats, were persecuted by the police, and were socially marginalised. They were gay, they were bottoms (and spoke in derogatory terms about tops), and they referred to themselves with feminine terms. Some of them were prostitutes, and some of them we would now refer to as trans women.
“Köçek troupe at a fair” at Sultan Ahmed’s 1720 celebration of his son’s circumcision.
Here’s a short documentary:
In Turkey a similar cant called Lubunca  was also used by sex workers and the gay “community” (as one says These Days); indeed, in the late Ottoman era it was spoken by the cross-dressing male köcek dancers. Based on Romani, it contains elements of Greek, Arabic, Armenian, and French.
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This leads us closer to (my) home with Polari, a British cant that has declined since the 1960s. Paul Baker has written two books on the topic.  Mixing Romance, Romani, and London slang, It was used by “some actors, circus and fairground showmen, professional wrestlers, merchant navy sailors, criminals, sex workers, and the gay subculture”; it’s said to have been used by Punch and Judy street puppet performers. Later Polari incorporated some Yiddish and 1960s’ drug slang.
- bona good (in Shakespeare! Unlike Philomena Cunk’s putative neologisms)
- ajax nearby
- eek face
- cod tacky
- lattie room (to let)
- nanti not, no
- omi man
- palone woman (from Italian paglione, “straw mattress”)
- riah hair
- rozzer cop (natural adversaries of the subculture, aka “Betty bracelets”, “lily law”, “hilda handcuffs”, “orderly daughters”).
- TBH “to be had”, sexually accessible
- zhoosht smarten up
- vada see.
I like arva, “to screw”, from Italian chiavare (cf. Burlesque-only’s immortal characterisation of Angela Merkel).
As in other secret languages such as that of blind musicians in China, numbers are interesting:
Among words that have entered the mainstream lexicon are
- slap [makeup]
- [rough] trade.
Polari minced into the wider public consciousness in the 1960s with Julian and Sandy on BBC Radio 4’s comedy series Round the Horne. I had little idea what it all meant, but that was kinda the point. There’s a clip on this page from Polari magazine.
As Paul Baker observes, after homosexuality was decriminalised in the UK in 1967, and as the gay liberation movement gained ground, the need for a secret language passed. While it was now associated with stereotypes often considered, well, naff, the camp image has maintained a certain frisson.
Here’s another bijou documentary:
 On Lubunca, the brief wiki article is augmented here; see also e.g.
 On Polari, some other sites include
and The Polari Bible.