Primarily an out-dated term, a carryout boy (or bag boy, also courtesy clerk) is a young man employed by a grocery store to bag your groceries after the cashier has "rung them up". Then, he places them (bagged) back into the cart, and follows you out to your car. He then places your groceries in your car, wishes you a good day, and takes the shopping cart back into the store to help the next customer.
With the rising popularity of large-scale grocery stores, an emphasis on downscaling, and worries about sexism, this has become an increasingly rare position/service.
A number of grocery chains now employ individuals to bag groceries, and then help those who ask or who appear to need assistance, only, while others such as Food4Less and Aldi's have the customer handle even the bagging, claiming that it helps them keep the prices low.