What kind of "prison slang" are there? I haven't heard of any.
"Prison slang" refers to slang terms that are used by inmates within a prison. There are many different slang terms used in prisons but most of them are unknown to people who have never been in prison. Most "prison slang" terms are related to criminal activity, prison life, and other inmates, most of which are not used in daily conversation by regular English speakers. However, some of this slang has been made more popular from the media. For the example, the term "bagman" comes from prison slang and refers to someone who is in possession of drugs.