David Ayer Gives his Take on Who Killed Robin, and How Joker Lost His Teeth
During the intro sequence for Harley Quinn, a blink-and-youll-miss-it line in her description described her as an "Accomplice to the murder of Robin," confirming that she's been around since before Robin perished at the hands of, presumably, the Joker.
"This is sort of my personal thing and maybe less about a larger connection," David Ayer said right off the bat. "But Joker killed Robin and Batman basically smashes his teeth out and locks him up in Arkham Asylum. It’s in the asylum where Joker would have done the ‘damaged’ tattoo as a message to Batman saying, 'You’ve damaged me. I was so beautiful before and now you’ve destroyed my face.' That’s where the grill comes from."
Whether this is elaborated on in a future film remains to be seen, but it would make for a hell of a Batman solo movie, bringing in Ben Affleck, Jared Leto, Margot Robbie, and a yet-to-be-cast Robin for a flashback story I wouldn't mind seeing.
Source: Comicbook