The story
From slan (“science fiction fan”) + shack. From a clubhouse for science fiction fans in Battle Creek, Michigan, USA called "Slan Shack" which existed from 1943 to 1945; from the fanspeak term slan; from the science fiction novel Slan (1940) by A. E. van Vogt, by analogy to the superior but persecuted mutants of the novel.
Etymology from Wiktionary, CC BY-SA 4.0.