water off a duck's back

大統領候補のLakeは対立候補との公開討論に臨んでいます。対立候補が先にスピーチをすることになりました。
By a flip of the coin, he was chosen to go first. He stumbled badly in his opening remarks as he moved stiffly around the stage, trying desperately to look at ease but forgetting what his notes said. Sure he'd once been a lawyer, but his specialty had been securities. As he forgot one point after another, he returned to his common theme-Mr. Lake here is trying to buy this election because he has nothing to say. A nasty tone developed quickly. Lake smiled handsomely; water off a duck's back.
最後に出てきた "water off a duck's back" は分かり易い比喩表現ですね。日本語でも糠に釘、馬耳東風とか色々ありますが、水からの連想で、蛙のつらに小便を真っ先に思い浮かべました。
英英辞書で意味を一応確認します。
・Cambridge English Dictionary: criticisms of or warnings to a particular person that have no effect on that person: I've told him that he's heading for trouble, but he doesn't listen - it's just water off a duck's back.
・McGraw-Hill Dictionary of American Idioms and Phrasal Verbs: 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.