If you see a good looking girl with a nice ass, plain and simple she is a "dump truck". Why? because you want this girl to come home with you and have her back it up and dump her booty on you. Meaning even more specifically intercourse using the doggy style position.
You: "DAMN DUDE!, check out Tracy!!!"
Friend: "Wow check her out, she's a dump truck!"
You: "I'm going to bring her home for some dump truck action tonight."