I know this means that if you were in a restaurant and didn't eat all your meal that you could ask for a 'doggy bag' for the rest of the meal so you can take it home. Where did it come from though?
The words 'doggy bag' first appeared in print in a magazine in the USA in 1964. The idea behind it was that the diners who took one away from a restaurant were taking the food home for their dog, whereas they were in reality probably intending to reheat it for their own benefit the next day!
As you probably know, the repressed british would die sooner than admit that they wanted to take food home, wheras in the U.S. it os perfectly acceptable and common practice - maybe they are just more at ease with themselves?
Here is the US, doggie bags WERE originally so restaurant patrons could bring home the meat scraps to their dogs. However, it has evolved into just asking for a doggie bag to bring them home for later human consumption as well. We are shameless and don't mind having a wait person bag-up our leftovers to eat later. If I am alone I often even ask them before hand to just put the salad (dressing on the side)in a 'to go' box since I know I will not be able to consume the entire meal.