battering ram: a ram used to break down doors of fortified buildings
engine: an instrument or machine that is used in warfare, such as a battering ram, catapult, artillery piece, etc.
mucus: protective secretion of the mucus membranes; in the gut it lubricates the passage of food and protects the epithelial cells; in the nose and throat and lungs it can make it difficult for bacteria to penetrate the body through the epithelium