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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。
criticisms of or warnings to a particular person that have no effect on that person
(批评或警诫对某人)像水过鸭背，毫无影响，不起作用
I've told him that he's heading for trouble, but he doesn't listen - it's just water off a duck's back.
我已经告诉过他会有麻烦的，可是他不听——说了也白说，根本毫无作用。