An unhealthy burger from a takeaway or pizza shop that somehow resembles something edible and tasty when drunk, but to the sober eye is blatently pure evil and not fit for human consumption!
Kerry: Joe, I fancy a dirt burger on the way home.
Sue (who is nicknamed Joe but is actually called Sue): Ooooh good plan batman. Garlic mayo or chilli dressing??
Kerry: Definately garlic mayo.
Sue: Hmmm, garlic mayo makes the best dirt burger.