Doughboy was basically an American slang term. It was used for American Infantryman especially during WW1. The exact origin of this term is not known however, it dates back to the American civil war. Many believe it came into use because of the dumpling shaped buttons on the Union Soldiers jackets. It is also believed that the infantry were covered in dust because they marched in the dry terrain during Punitive expedition, which gave them the look of unbaked dough.