like water off a duck's back

Fig. easily; without any apparent effect. Insults rolled off John like water off a duck's back. The bullets had no effect on the steel door. They fell away like water off a duck's back.

Readily and without apparent effect. For example, The scathing reviews rolled off him like water off a duck's back. This expression alludes to the fact that duck feathers shed water. [Early 1800s]

Without any apparent effect. Ducks' feathers are waterproof. The preen (or, formally, the uropygial) gland at the base of the tail produces oil that spreads and covers the birds' outer coat so that water forms droplets on, but does not permeate, the feathers. That's why a critical remark that doesn't bother the person for whom it was intended rolls off like water off a duck's back.