back when Britney Spears was still hot
Used as an exaggeration for a time period between something that just happened and the last time that same thing happened.
Matt: I'll beat you at ping pong again, like I did last time.
Caleb: Matt the last time you beat me at ping pong was back when Britney Spears was still hot.
Caleb: Matt the last time you beat me at ping pong was back when Britney Spears was still hot.