Green’s Dictionary of Slang

canon adj.

1. (juv.) that which is true to the original story; thus headcanon, that which isn't thus true, but has been created for an individual's Alternative Universe version of that story.

Urban Dict. 16 Oct. 🌐 My AU was not canon to the original plot in the movie, but I think people will like this spin on the story [...] They had made their own head-canon, where Lucy had survived the explosion.
Axis Parents’ Guide to Teen Sl. 🌐 Canon/Headcanon = The actual plot of a book, movie, or show (canon) vs ideas people have that they want to be canon (headcanon).

2. see cannon adj.