Probably a clipping of althistory, itself a clipping of alternate history. Analysable as alt + hist.
althist (plural althists)
- (Internet slang, fandom slang) Synonym of alternate history (“a genre of fiction based around or involving ahistorical events”)
1997 October 10, Cairo, “Alt History Book Reviews”, in alt.history.what-if[1] (Usenet):I read the following: […] there's a book of micheal moorcock on althists, which isn't too good
2010 April 8, Lord_Vexacus, Twitter[2]:Turtledove is the master of AltHist. I just wish he'd do a decent Japanowank series
2023 June 9, u/TechnologyBig8361, “What's a (real) country that sounds like it was created for someone's althist project?”, in r/AlternateHistory[3], Reddit:[see title]