Nowonmai, from the 1971 novel by William Peter Blatty, is the reverse English for "I am no won (one)". It is the response the demon gives to psychiatrist priest Father Damien Karras when he asked for his name. Much of the dialogue in that scene from the demon is reverse English.
In the film version, released in 1973, the young actress Linda Blair gives a wrenching performance as Regan, with Ellen Burstyn as her mother, Chris, and Max von Sydow as Father Merrin. The demonic voices were provided by the actress Mercedes McCambridge, and the theme music, “Tubular Bells,” was nominated for an Oscar. Audiences were traumatized by the film, and some persons sought professional help in the fear that they might become possessed themselves. - Encyclopedia of Demons and Demonology. Rosemary Ellen Guiley, 2009
The terms demonology and demonologist are two of the most misused terms in the world of pseudoscience, and sometimes even within evangelical belief systems centered around modern interpretations of exorcism.
A cursory search online reveals several “demonologists” and their work. Some appear in sensationalist photos displaying various wares as though taking on a pseudo-witch hunter role against “the Devil” and “demons,” though they rarely explicitly define what they mean when using these terms.
Find out what's the truth behind the word "demonology" from Stanley Stepanic's Demonology: A Study of What Is Not.