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.
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.
FRIEND A: "Did you just take a stealthie of me?"
FRIEND B (turning phone around): "no I was just using snapchat's new filter, see?"