http://www.slate.com/articles/news_and_politics/crime/2015/07/prison_slang_prison_inmates_in_missouri_got_together_and_made_a_lexicon.html

Prison slang is forged under pressure. A word that means one thing on the outside can mean something entirely different when used by people leading highly restricted lives in cramped, often dangerous facilities. Inmate shorthand can be indecipherable even to the family members who come to visit them, and it can vary greatly from one institution to the next.