雌黄

雌黄 is an actual yellow mineral, arsenic trisulfide (As2S3). Ancient Chinese scribes used it as an early correction fluid: dabbed onto bamboo or yellow paper, it covered miswritten characters. From this came the famous idiom 信口雌黄 - literally "orpiment from the mouth" - meaning "to spout corrections irresponsibly," hence "to talk nonsense / slander baselessly."

Don't confuse with 雄黄, which is a different arsenic mineral, realgar (As4S4, red-orange). 雌 (female) vs 雄 (male) - same pattern as 雌雄 (female-and-male).