当头一棒

Explanation: Head-on a stick. Metaphor seriously warning or a sudden blow. 　　Usage: Partial official; as predicate, object; metaphor seriously warning or a sudden blow 　　Source: Cao Xueqin, "Dream of Red Mansions" at 117 back: "A monk asked Yu to hear that, like blow, he said: 'You do not have money, I put the jade also you stop'." 　　Examples: Confucius seems to be a ~, soul sat lost soul, just as a stay of wood. (Lu Xun, "Stories speak out" Thesaurus:  a sharp warning, Bell Drum