Dem Deg is substitute for saying the commonly used expression "Damn Dude." It can be used in almost any situation, and is most frequently used when one has no other words to express himself. In short, Dem Deg can be used to mean almost anything, if you follow the context.
Lad 1 - Coming out tonight for a a few Sheg Deg's?
Lad 2 - A few?! Nah I want to get proper Degged up! I've already booked myself in for a stomach pumping at Worcester hospital!