A term given to Marines by the Germans in WWI at the Battle of Beleau Wood in France. During the battle, the Marines charged into the German trenches through mustard gas (which made their eyes red), and faught with such ferocity that the dying Germans called them devil dogs. The term is still used today between Marines as a motivational nickname.
"Semper Fi, devil dog!"
"Hey there devil dog, you keep shooting like that and there won't be any enemy for the rest of us."