Although the duck spends a lot of time in water, the water never stays on its feathers for long. This is because its feathers are coated with oil and this prevents the water from being absorbed. As a result, when the bird steps out of the water, it is able to shake it off within a matter of a few seconds. Spending a considerable amount of time in a pond or a lake seems to have no effect on the bird. Similarly, when you criticise someone and it has no effect on him/her, then you can say that the criticism was like water off a duck’s back.
Example: I have scolded Achala several times for being lazy and sloppy, but it’s just water off a duck’s back to her.