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]
American Heritage Idioms
Phrase(s): 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.
McGrawhill's American Idioms And Phrasal Verbs