water off a duck's back鸭子身上的毛就像雨衣一样，水在上面呆不住，一下全滑掉了。因此，water off a duck's back 是指毫无作用，对牛弹琴。

【例句】
Their hints about his behaviour were water off a duck's back.
他们示意他举止不当，如同对牛弹琴。

I can't tell my son what to do; it's water off a duck's back with him.
我无法告诉我儿子该做什么，他根本听不进去。