A male sex organ (penis) so long that hypothetically, while walking outside it would drag in the dirt.
Person 1-"hey man wanna go to the beach with the gang today?"
Person 2- "Unless its a nude beach I cant"
Person 1- "Whys that?"
Person 2- "Because I have a dirt dragger"
Person 1- "What's that mean?"
Person 2- "It means that my dick is so huge that it hangs out of my shorts and drags on the ground"