Meaning of “be (like) water off a duck's back” - Learner’s Dictionary

be (like) water off a duck's back

If criticisms, insults, etc are like water off a duck's back to you, they do not affect you at all:

She calls him lazy and useless, but it's like water off a duck's back.

(Definition of “be (like) water off a duck's back” from the Cambridge Learner’s Dictionary © Cambridge University Press)