A term used for fan fiction in which aspects of the canon (original universe) are so grossly exaggerated as to be unrecognizable except in name--sometimes offensively so.
This can happen to characters who the author worships or loathes; to a plot/story arc the author wishes to change/"fix"/invalidate; to a shipping (character relationship) that the author favors or disapproves of; or even to the entire canon universe, if the author's philosophy and "sense of life" differs greatly from the creators' sensibility.
Note that calling something "canon rape" does not necessarily mean it's badly written...but since the term is considered pejorative, people tend to associate it with badfic.
Oh god, I just read that new Star Wars story that got posted. The way they demonized Yoda and the Jedi Council, and excused all of Annakin's murders, it was complete canon rape.
Yeah, but it was kinda well written. If the author would just change the names and some of the background stuff, it'd make a pretty good original story.