You haven't got a letter on yours,"George observed. "I suppose she thinks you don't forget your name. But we're not stupid-we know we're called Gred and Forge.

In this quote, George is saying that he and his friend (presumably named Gred and Forge) have not received a letter addressed to them, and he assumes it is because the person who sent the letter believes they already know their own names and do not need a reminder. He implies that Gred and Forge are intelligent enough to remember what their names are.