The phrase like water off (also from) a duck’s back means: with no effect or reaction.
This phrase occurs, for example, in Peace at last for Saddam look-alike George, by Karl Connor, published in the Sunderland Echo (Sunderland, Tyne and Wear, England) of Thursday 18th December 2003 [page 7, column 2]:
ONE man is sleeping much easier since U.S. troops captured tyrant Saddam Hussein—his lookalike George Wilson.
[…]
The capture of the real Saddam has not stopped regulars at George’s local, The Willow Pond in Hylton Road, from giving him stick.
“I went for a pint on Sunday night and the lads were all like ‘didn’t think they’d let you out this fast’, but I’ve have had it for years from them so it’s like water off a duck’s back.”
The earliest occurrences of the phrase like water off (also from) a duck’s back that I have found are as follows, in chronological order:
1-: From Dialogue V. Between Farmer Littleworth, Mr. Brisk (Mr. Dolittle’s Curate), Mr. Smirking (Assistant to Dr. Dronish), and the Farmer’s Family. On the evil Nature and Effects of Stage Plays, published in Village Dialogues, between Farmer Littleworth and Thomas Newman, Rev. Messrs. Lovegood, Dolittle, and others (London: Printed for T. Williams, 1801), by the English preacher Rowland Hill (1744-1833) [second edition, page 64]:
F. In my vain days, when I attended these places, I know not what I, or any one else, went there for unless to make game, and to kill time, as we most wickedly used to call it. The Lord knows, that bad I went there, and worse, I am sure, I came away, for, as to all the wicked things I heard there, they stuck to my heart like burrs to my worsted stockings, and as to the morality, that all ran off as fast as it came on, like fair water from a duck’s back.
2-: From Maxims of Sir Morgan ODoherty, Bart. Part the Third, published in Blackwood’s Edinburgh Magazine (Edinburgh, Midlothian, Scotland) of September 1824 [page 347]:
Maxim One Hundred and Thirty-Sixth.
If you be an author, never disturb yourself about little squibs, &c. against you. If you do, you will never be at rest. If you want to annoy the squibber, pretend never to have heard of them. It is only five days ago since I was in company with Rogers and Tom Moore, and no pair could harmonize better.—Yet who does not know Tom’s epigram on Sam? Rogers had made him a present of a copy of Paradise Lost, in which there was the very common frontispiece of the devil in shape of a serpent, twining round down the tree of knowledge, with the fatal apple in his mouth, which he was in the act of presenting to Eve; and under it Tom, instigated no doubt by the evil spirit, whose picture he was inspecting, wrote—
“WITH EQUAL GOOD NATURE, GOOD GRACE, AND GOOD LOOKS,
AS THE DEVIL GAVE APPLES, SAM ROGERS GIVES BOOKS.”
An unkind return certainly for civility. The cut at the looks was particularly unfair, as Mr Rogers is a bachelor; but he only laughed, as he always does, and the thing passed off like water from a duck’s back.
3-: From a review of The Gamester, by the English playwright Edward Moore (1712-1757), produced at Drury Lane, London—review published in The Examiner (London, England) of Sunday 23rd November 1828 [page 756, column 1]:
[Beverley’s wife] is so exquisitely sweet and gentle a creature, that one looks upon her rather as one of those “blest spirits made perfect,” than as a denizen of our own sphere. Come what misfortune may, her great and inestimable love seems to throw it off like water from a duck’s back: neglect, ruin, unmitigated poverty, create in her a higher feeling than that of resignation, she is almost cheerful.
4-: From a review of Library of Entertaining Knowledge. Part IV, published in The New Monthly Magazine and Literary Journal (London, England) of November 1829 [page 469, column 2]:
Of the collection of examples, […] the numbers overwhelm, and the generality of the phrases, quite inevitable with such multitudinous examples, must slip away from the memory, like water from a duck’s back, and work no effect.
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