The term 'Mistress of Gangster' can be summarized as a 'Moll', which is a four letter word. A Moll is a female associate of a gangster, typically one who is involved in a romantic or sexual relationship with the gangster. The term originated in the early 20th century and was used to describe a woman who was involved with a gangster, often providing them with money, shelter, and other forms of support.