Be water off a duck's back - Idioms by The Free Dictionary
be water off a duck's back
be (like) water off a duck's back
To be a fleeting presence; to not have a lasting impact on someone or something. I envy my sister because any criticism or judgment is like water off a duck's back to her.
be (like) water off a ˌduck’s ˈback (informal) used to say that something, especially criticism, has no effect on somebody: His book got bad reviews, but it was all water off a duck’s back — he doesn’t care at all what they say.