Here from metafandom. My guess would actually be from within fandom -- on the Lois McMaster Bujold List. I joined back in the late 90's, and it was well in use (with that definition) then. I have a vague recollection that someone on the list started it, but I can't know for certain.

It's actually unlikely to come from Holmesians and Sherlockians, because from everything that I've read about them, they work *only* from a Watsonian perspective. ("The Game," as IIRC Leslie Klinger put it.)