There was man name jack Napier and after his wife died . He decided to join the mob and so the mob and Andrea father made the deal if he gets the money he lives so after that jack decided to kill Andrea father so. If he want to avoid people know he killed him so some of mob member got him the red hood mask with a red cape . And their plan is for him to steal 10,0000$ but it failed when the police shot some of the mob member so jack needed to escape so he got on the bridge but as soon as batman showed up . Jack want to get away from batman so he jump and dive into a lake of experimented chemicals so after he got on land and go back to his apartment . He took off the hood . When he needed to go to wash the chemicals off . He felt his skin burning and decided to took a shower but before he did he check the mirrors and found out his skin had permanently beach so he said " I look like a evil clown, my skin chalk white,my hair is green and my lips are ruby red and what a joke on me!" So he started to scream and went insane . And found out he became .......the joker.
So this is my version of his origin story since they never gave him one . Gosh people made so many origins