Xin Kou Ci Huang (Randomly Speaking Out Orpiment)
"Orpiment" is a kind of reddish-yellow mineral which was used in ancient times to paint over mistakes one made when writing.
During the Jin dynasty, there lived a man named Wang Yan, who was very eloquent. Wang Yan worked for the government, and would often travel from place to place lecturing the affairs of the country. His lectures were very thorough, and he also added in his own opinions, so his audience always understood him, Wang Yan was a very well-known lecture, admired by everyone.
Whenever Wang Yan gave a lecture, he was always at ease and unhurried. If he made a slip of tongue,he would calmly go back and correct himself. Therefore people said that the "had orpiment in his mouth".
People of later generations expanded this idiom. Randomly saying things for which one has no proof is now known as "randomly speaking out orpiment."