Polari, or "The Lost Language of Gay Men" as Paul Baker has it in his wonderfully readable lexicon of that name, is the secret parlance through which gay men secretly communicated with each other during most of the 20th century.
According to a recent article in the Guardian ("What brings you trolling back, then?" by Colin Richardson), it flourished between Oscar Wilde's trial in 1895 and the decriminalisation of homosexuality (in England) in 1967.
Polari-speak is not so much "lost" now, as part of our common culture—witness "cruising", "cottaging", and even the most basic gay word, "camp". By the way, the next time you "cold-call" someone, looking for a positive outcome, you are speaking Polari.
Discovered deep within Evan Zimroth's fascinating read about the coded sex-diaries kept by the Economist, John Maynard Keynes at MoreIntelligentLife.com.
(Not recommended for the easily offended.)