water off a duck's back

Etymology
From the idea that water rolls easily off a duck's back and does not penetrate the duck's feathers.

Noun

 * 1) Something potentially harmful or hurtful, such as criticism or insult, that nevertheless has no effect or impact on the target.
 * She's always going on at him about something, but to him it's all water off a duck's back.

Usage notes
Often in the expression like water off a duck's back.