Headcanon is fandom slang for elements or interpretations of a (usually) fictional universe accepted by an individual fan as being canon. It may or may not be supported by evidence from the actual work (which is where Wiktionary gets it wrong). You could almost think of some headcanons as being literary theory for fun. Everybody has a headcanon for something, and those of us who get crazy with fandom cheez-wiz tend to have a lot of them.
My favorite headcanon comes from comics, specifically the works of Grant Morrison. In the late 2000s and early 2010s, he was wrapping up something like twenty years of storytelling that had carried through at least a half dozen series and major events, culminating in Final Crisis and The Return of Bruce Wayne.
In short: DC has a guy named Darksied. He’s the New God of Tyranny. Jack Kirby invented him, Grant Morrison rescued him from the scrappy heap and made him the ultimate Big Bad of the DC universe. He has an ability called the Omega Sanction. If you get hit with it, you suffer the so-called ‘death that is life’ – endlessly living and dying in progressively more horrific circumstances. Give Morrison’s Seven Soldiers of Victory a read sometime, as it shows the Omega Sanction in all its nightmarish glory. In Final Crisis, Batman got hit with it and seemingly died. Except not. It involved time travel and shenanigans.
This is where headcanon comes in.
The Omega Sanction produces a hyper-adaptive killer curse. You beat it once, it changes to overcome you on the next go round. You lose to it once, it changes to make you lose harder next time. It can move whole centuries around just to mess with you. You cannot win. You will not win. It will kill you again, and again, and again. This will continue until long, long after you break. It is, in fact, supposed to go on forever.
With this in mind: My headcanon is that the Joker used to be a hero. A full-blown, goody-two-shoes good guy. Maybe he didn’t have much of a sense of humor, maybe he was a wisecracking snarker like Spider-man, maybe he was just unrecognizable. Point is, he was heroic. He stood up to Darksied and got hit with the Omega Sanction for his troubles. Slowly, over the course of lifetimes, it broke him down and transformed him into the reprehensible monster that we all know and love.
But the Joker, being a hero, did not go quietly. He went down fighting to the bitter end, and maybe even beyond that. His last great deed was to corrupt the corrupter, to redeem the very thing that was destroying him. He converted Darksied’s hyper-adaptive killer curse into a weapon against him, against tyranny and injustice. He made a hero out of a nightmare and sicced it on the forces of evil.
This is why Darksied’s Omega Sanction failed when he threw it at Batman. Unbeknownst to him thanks to a few cosmic resets, he was trying to use one hyper-adapter to kill another, and the one with more experience won out. This is also the real reason why Batman, in most iterations, will never kill the Joker. Deep, deep, deep down, beneath everything human, the entity that became the basis of Bruce Wayne still remembers what the Joker used to be. It remembers what it did to him, and it knows the real reason why he is the way he is. Its guilt is a check on anything Batman could ever do to stop his oldest enemy – his first friend.
This also explains why Batman won’t kill in general, and it provides a neat reason why the Joker seems to mostly limit himself to Gotham City. It even explains how the Batgod/Prep Time meme. Somewhere beneath layers of abstract insanity, the heroic core of the Joker is still struggling to at least limit the collateral damage he’s truly capable of. It even explains the Joker’s infamous multiple choice past – they’re all true, they all actually happened, and he remembers all of them. Heck, maybe Batman is still carrying out a form of his original programming and the Joker is actually still subverting him even now, forcing the hyper-adapter to be supremely heroic by being a villain. In his own way, I like to think the Joker has become a sort of junior New God, a being similar to Darksied himself. His portfolio consists of insanity, degradation, and change.