The term 'Whodunit Terrier' is a reference to the 1934 novel 'The Thin Man' by Dashiell Hammett. The novel follows the adventures of a married couple, Nick and Nora Charles, as they investigate a murder mystery. The novel was adapted into a film in 1934, and the character ASTA, a wire-haired fox terrier, became a popular figure in the film. ASTA is often referred to as a 'Whodunit Terrier' due to his role in helping Nick and Nora solve the mystery.