The word 肥皂 (féizào), meaning "soap", is formed by combining two characters that historically describe its core ingredients and appearance. 肥 (féi) means "fat" or "fatty", referring to the animal or vegetable fats used as the base. 皂 (zào) originally meant "black" or "dark-colored", likely describing the dark, ash-based substance (from burnt plants like soapwort or beans) that was combined with the fat to create the earliest cleaning agents. Thus, the term visually and materially evokes the traditional recipe of "fat" mixed with a "dark alkali".