皂 originally referred to
皂斗 (zàodǒu) — acorns/oak nuts whose shells were boiled to produce a black dye. From 'black' the meaning extended to soap (which historically often was dark). The famous chengyu
不分青红皂白 (bù fēn qīng hóng zào bái, 'not distinguishing greens, reds, blacks and whites') means acting without considering right and wrong.
