The word 病入膏肓 is formed through a literal sequence of four characters: 病 (illness), 入 (to enter), 膏 (the fat or grease between the heart and diaphragm), and 肓 (the membrane between the heart and diaphragm), with 膏 and 肓 together denoting a critical anatomical region in traditional Chinese medicine. The combination directly describes an illness penetrating into this deep, inaccessible area, resulting in a fixed phrase that conveys a severe, incurable state.