信口雌黃　

◎ 信口雌黃 xìnkǒu-cíhuáng

[make irresponsible remarks;talk sheer nonsense;wag one‘s tongue too freely] 古時寫字用黃紙,寫錯了就用雌黃涂抹改寫。比喻不顧事實,隨意亂說