The threesome prison deal is a popular term used in the adult dating world to describe situations in which two (or more) women are in jail together. They are also known as the "three-letter" word, where the letters stand for "threesome". The three-letter word has gotten so much exposure lately because of the controversy and interest surrounding it. For many years now, public debate about the definition of the word "threesome" has raged in the public arena. Many people believe that the term refers to a sexual encounter in which the man penetrates his woman from behind while she is lying on her back. Others believe that it refers to the sexual position in which a woman is placed by her partner and is sexually penetrated by him from behind..