The origin of the term can be traced back to the New Testament of the Bible. In both Gospel of Mark and Matthew, Jesus condemns those who have been using the sacred temple as a marketplace, proclaiming that they have "made it a den of thieves." To this day, writers use the word "den" to mean a place where iniquitous activities are carried out.
- (idiomatic) A place of immoral behavior, usually of a sexual type.
The song "In Our Little Den of Iniquity" from the 1940s musical "Pal Joey" is an example. It refers to a secret hideaway for romantic rendezvous. Used more solemnly, the expression "den of iniquity" implies a certain discernment that a morally upright person would consider intolerable. There are several earlier (19th century) examples viewable on the Making of America websites, such as this one from an 1840 book by George E. Dabney: "The young men made their preparations with alacrity, and, headed by this western patriarch, we proceeded at a rapid gait to surprize the tenants of this den of iniquity."