The character "Stopthief' is a child who does not remember his past. He essentially grew up on the streets and had to fend for himself. This included taking food from others because he obviously had no money with which to pay. He does not remember a mother or father, a home, or a name. The earliest sense of identity came when someone called, "Stop thief!" As a result, he believes that "Stopthief" is his name.
Because all the Jackboots capt calling him that and he thought that was his name!