There is no canonical story when it comes to the origins of The Joker, but one of the best and most appreciated by fans is the story presented in the graphic novel “The Killing Joke.” In this novel, The Joker was originally a failed comedian who became a petty criminal because his wife was pregnant and he was struggling to support her financially. While in a meeting at a bar with mobsters, police show up and tell him that his wife died in a sudden freak accident. With his wife and their unborn child both dead, the comedian (who was not yet The Joker) attempts to back out of his deal to help the mobsters rob a chemical plant, but they threaten his life and force him to go along with the scheme as planned.
During the heist, the comedian is dressed in a red hood as a disguise. When Batman shows up to stop the heist, the comedian accidentally falls into a vat of chemicals. When he emerges, his skin is stained white, his lips red, and he has a permanent smile on his face. Having lost everything, he has also been driven completely insane.
The story goes on to brilliantly describe The Joker’s way of thinking. He kidnaps shoots Barbara Gordon in the spine, paralyzing her, and then kidnaps Commissioner Gordon and attempts to drive him crazy by forcing him through a sadistic theme park ride in which he is forced to see pictures of his injured daughter. The Joker maintains throughout this that all it takes is “one bad day” to turn a perfectly sane man as insane as he is.
This Joker backstory is not considered canonical, as The Joker is not a reliable narrator, even stating in the book that he remembers many versions of his origin story.