What is Polari?

Definitions from Oxford Dictionary

Polari

/pəˈlärē/

noun

a form of slang incorporating Italianate words, rhyming slang, and Romani, used originally as a kind of secret language by people in theatres, circus preformers, mariners and seaman, prostitutes, criminals, professional wrestlers etc. and adopted by some gay people in the 20th century.

What is Polari and it’s history?