The Philadelphia Dump Truck is a made up sexual term from the popular comedy show, Two and a Half Men, Season 10 Episode three, "Four Balls, Two Bats and One Mitt". It is not clear what the term means since its not a real sexual term and is not explained in the show.
Since a dump truck is defined as, a truck having a load-bearing container at the back that can be tipped up to unload the contents from the rear of the vehicle, this term most likely has something to do with the contents of someone's anal cavity being "unloaded", of possibly semen, sex toys, or even fecal matter. Since semen and sex toys can be loaded and unloaded from the anus as with a dump truck, I'm guessing its one of those two. The significance of "Philadelphia" is unclear. The word was either picked at random, or is possibly an inside joke or has some unknown significance among the show's cast/production team.
Alan, "We need to establish some ground rules.
Walden, "Sure, like what?"
Alan, "Ah, well um, for instance uh, there are a few things that I happen to know that Lyndsey is not exactly fond of doing."
Lyndsey, "No, I'm good." *smiles*
Alan, "Re..really? Your good?"
Alan, "....With everything?"
Lyndsey nods again.
Alan, "Even the Philadelphia Dump Truck?"
Lyndsey leans over to Walden, "Back it up!", then faces Alan, "Beep beep beep beep."