皂

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.

On its own 皂 is rare in modern speech; most often you'll see it in 肥皂 (féizào, soap), 香皂 (xiāngzào, scented soap / bar soap), and the idiom above.