The story
From Watson + -ian. The “in-universe” (sense 1.1) sense formed in reference to Dr. John Watson, the fictional author within the text of the Sherlock Holmes stories, in contrast to Arthur Conan Doyle as the real author of the stories (compare Doylist). The term was coined in the Sherlock Holmes fandom before spreading to other fandoms. It possibly originated in the The Baker Street Irregulars fan club and entered science fiction fandom slang via the Lois McMaster Bujold mailing list.
Etymology from Wiktionary, CC BY-SA 4.0.