When was this taken? Were they hooking up? Why is Leo rocking a Boston Red Sox cap while throwing up a west coast sign? So many questions!

On Thursday Britney Spears posted the picture above of herself partying with Leonardo DiCaprio and simply captioned the shot, "#tbt." Killing us, Brit Brit!

It's not clear when the pic was snapped, but both of the A-listers look pretty fresh-faced -- and like they're at a club, so we're going to go ahead and guess this was at least ten years ago. Then again, Leo still parties and Britney's been looking better than ever, so who really knows!

Wonder what Leo thinks of this?