Reminiscent of the hunched over cave man, with his arms to the ground. An insult used against those of extremely low intelligence, or general stupidity.
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"