Literally, to consume a bag full of food from Dick's Drive In, usually 5 fries to one burger.
In Seattle, "Eat a bag of Dicks" might be taken as a lunch suggestion instead of a deadly insult.
Frank: Damn I'm hungry, let's break for lunch.
Paul in Accounting shouts out: Hey frank, go eat a bag of Dicks!
Frank: Sure, I haven't had Dicks in a while, there's one on Broadway - great fries!