(like) water off a duck's back idiom
C2 （批评或警诫对某人）像水过鸭背，毫无影响，不起作用
criticisms of or warnings to a particular person that have no effect on that person
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。