Polari is a sort of slang once widely used in England by sailors, theater folk, and gay people. Polari is a cant, another term I've only just learned, which is a slang language meant to exclude outsiders.
The word comes from the Italian word parlare, which means to talk. It is fitting that the word has its origins in Italy, because Italian is one of Polari's many ancestors. Polari is heavily influenced by the language of Italian sailors and showmen who went to England to work. Sailors often worked in the theatre, as handling curtains and riggings and such is very similar to handling sails and riggings and such. Gay folk have always been a big part of the theatrical community, so it is understandable that Polari would be common among them. Other languages that contribute to Polari include the other romance languages, Romany, Yiddish, Cockney, and Shelta, the language of Irish tinkers.
Here's an example of Polari in action:
It is funny that the two men are discussing A Clockwork Orange because, while Alex and his droogs don't speak Polari, they do speak their own criminal cant throughout the book.
Although the video is fairly incomprehensible (there's a translation here), some of the slang terms used in and out the gay community come from Polari.
- Butch: A gay woman who isn't considered feminine.
- Bitch: A gay man who is considered feminine.
- Drag: Originally used to just refer to clothing, came to refer specifically to cross-dressing over time.
- Fruit: A gay person - this term didn't take on its pejorative meaning until later.
- Queen: A flamboyantly gay man.
- Camp: Deliberately over-the-top theatricality, possibly from the French camper, which means to pose.
Other words, like hoofer (dancer) and filly (young person), have found their way into English proper.
Sadly, the following words have fallen out of popularity: ajax (next to), batt (shoe), lacoddy (body), and yews (eyes).
Info from http://www.worldwidewords.org/, this Slate article, and http://www.ling.lancs.ac.uk/staff/paulb/polari/home.htm, http://www.etymonline.com/