A phrase used in response to someone starting a new thread/conversation/tweet/etc... about a topic/website/video/etc... that was discussed ad nauseum by the rest of the civilized world a number of months/years ago.
The phrase is derived from the Back to the Future trilogy of movies which involved a DeLorean that could travel through time. The plot of the first movie in the trilogy revolves around the main character becoming trapped in the past when he can't refuel the DeLorean.
Reference to Philadelphia's location and status as the nation's second largest burned out ghetto/city. Detroit is clearly the reigning champion of ghettoness.
Chad - I'm going to Philly on business next week.
Brad - Be sure and take body armor...they don't call it Detroit on the Delaware for nothing.
The last form of slavery in the US. This is where many young people begin careers and work 115 hours a week until they either quit or die from exhaustion. Former Deloitte employees often have scarred backs from the whip marks.