Shelta is one of the languages of the Irish travellers. Sometimes it's called the cant, and in Ireland it's called Gammon.
Shelta is a creole - a mixture of languages that's taken on a life of its own.
The ingredients of the mixture in the case of Shelta are Irish-English, Gaelic and Irish, with some borrowed Romani words, too.
Shelta may have been spoken since the 1200s, but the mix of languages in it has changed a lot, with the trend towards more English. Nowadays Shelta is mostly English, but it has thousands of extra words. Many of these extra words are at root Irish (often reversed, or with deliberate variations such as added syllables).
This, together with a custom of speaking very fast, means that Shelta can be used as a code language if necessary, though Shelta has a life of its own, and isn't only used to exclude outsiders.
The borrowing of words hasn't been entirely in on direction: one British English word, bloke, meaning man, probably comes from a Shelta word for boy.
Word To Use Today: a disguised word like the ones sometimes used in Shelta. That might involve reversing a bit of a word and then saying it fast: nobkers instead of bonkers, perhaps.
The word Shelta first appeared in print in 1882 in Charles Leland's book The Gypsies. It probably comes from the Irish word siúl, which means to walk.