Gutterspeak

Gutterspeak, sometimes called Thieves' Cant, is a language spoken by the denizens of shadow across Azeroth. Thieves, assassins, rogues, and spies and other individuals of ill repute use this language to communicate in secret ways. It involves a secret mix of dialect, jargon, and code that allows you to hide messages in seemingly normal conversation. Only another creature that knows gutterspeak can understands such messages. It takes four times longer to convey such a message than it does to speak the same idea plainly.

Those who can speak this language can also understand a set of secret signs and symbols used to convey short, simple messages, such as whether an area is dangerous or the territory of a thieves' guild, whether loot is nearby, or whether the people in an area are easy marks or will provide a safe house for thieves on the run.