肥皂
féi zào · HSK 7
soap
🧼 肥 = fat/grease + 皂 = soap-berry tree: grease-cutting soap-berry
Ancient soap was made from soap-berries (皂荚) mixed with animal fat (肥). The character 皂 originally meant the soap-berry tree. So 肥皂 literally recalls the old recipe: fat + soap-berry = soap.
肥 — flesh radical 月(肉) + 巴
Originally written with 肉 (flesh/meat) on the left and a phonetic-like component on the right, 肥 depicted abundant flesh on a body, hence 'fat' or 'plump'. Over time the 肉 radical was simplified to 月 in left-side position. The meaning extended from animal fatness to 'fertile' (as in rich soil) and 'fertilizer'.