Wiktionary
a. (context simile colloquial English) Without immediate or lasting effects. adv. (context simile colloquial English) In a manner which has no effect; immediately and without causing any difference.
Usage examples of "like water off a duck's back".
They shed the hardships of camp life like water off a duck's back.
All the stiffness and anger slid from him like water off a duck's back and his eyes, his face, his very person seemed suddenly aglow with excitement.
Mam'zelle still had a lot to say, but the words rolled off Kathleen's head like water off a duck's back.
The door swung open like water off a duck's back and there, framed in the portal, stood the most beautiful woman I'd seen all day.
Anything I've got to say on the subject rolls off her like water off a duck's back.
Jackie had tried her best to ignore it, to let it run off her like water off a duck's back.
I mean, because, you see, it exceeds imagining, it is beyond your imagining -- even when you're looking at it, you attempt to pour the salutary waters of description over these transdimensional objects and it runs, language runs off them like water off a duck's back.
Trisha wondered if she could do that-just let the shakes run off her like water off a duck's back and be still and conceal the churning inside her.