The story
From Doyle + -ist, with reference to Arthur Conan Doyle, the author of the Sherlock Holmes stories. Adjective formed by analogy to Doyle as the real author of the text, as opposed to Dr. John Watson as the fictional author within the text. Started in Sherlock Holmes fandom before spreading to other fandoms. Possibly originating in the "Baker Street Irregulars" fan club and entering the science fiction fandom usage (fanspeak) via the Lois McMaster Bujold mailing list.
Etymology from Wiktionary, CC BY-SA 4.0.