In other words, "My backstory is whatever you want it to be".

If you were the DM and this bothered you, the player just gave you powerful ammunition.

You could even have it so whenever the player entered a shop in his home town, the shopkeepers looked at him with disgust and refused to serve him. The DM wouldn't even have to necessarily come up with a reason. Just, that the player is extremely well known among the locals and they universally think he's absolutely disgusting and want nothing to do with him.