In The Dark Knight, the Joker describes a scenario in which his father once came home, attacked his mother, and then brandished a knife at him, all the while repeating the line “Why so serious?”
Is this story true?
There's no clear answer as to how the Joker got his scars in The Dark Knight. In the movie he gives two stories.
This is a reference to Batman: The Killing Joke where he sees his past as being multiple choice.