Argot

A secret or made-up language/code used by various groups, such as schoolmates, outlaws, colleagues, gay people, etc. to prevent outsiders from understanding their conversations. Typically considered a slang or a variation of style on an already existing language.
Some examples include: Nadsat, Eggy language, cockney rhyming, and pig latin.

"We need to create an argot so they don't know what we're saying."