a) someone who washes dishes for a living, in a cafe / bar etc.
b) term meant to be used as an insult.
a) Tony= "Man I fucking hate being a dish rat at the local pub. It's shit pay, and I come home smelling like shit"
Anothony= "Make your job sound better, call yourself an "Underwater Porcelain technician"
b) "Dude, your a fucking dishrat. Go use your n00b tubes some more.