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The expression sitting duck designates an easy target—i.e., a person who, or thing that, is defenceless against an attack or other danger.
This expression is a metaphor from duck-hunting, a sport in which shooting at a sitting duck is deemed dishonourable—as mentioned, for example, in the following paragraph from A Few Cracks from an Old Egg, published in The Rosalie Rip-Saw (Rosalie, Nebraska, USA) of Friday 23rd June 1933 [page 4, column 2]:
Most of us duck hunters agree, you should never shoot at a sitting duck, and we’ve always believed that shooting folly as it flies is certainly more sportsmanlike than waiting for it to roost.
—Cf. also the phrase like shooting fish in a barrel.
These are, in chronological order, the earliest occurrences of the expression sitting duck that I have found:
—Note: The first three occurrences, in the forms as doomed as a sitting duck and doomed like a sitting duck, are from texts by the Canadian journalist Gordon Sinclair (1900-1984):
1-: From the column Hook, Line and Sinclair: Stray Slants on Current Sport, by Gordon Sinclair, published in The Toronto Daily Star (Toronto, Ontario, Canada) of Wednesday 11th November 1936 [page 15, column 3]:
The biggest rugby league in Canada; the metropolitan college loop, dives into its play-offs to-morrow […]
[…]
[…] If they win this time it will be their fifth championship in a row and any expert who pays them off as has-beens has gone a bit punch goofy.
Len Thompson, their hoofer, can out-kick anybody on the big red squad; Haigh and Wetton can pass and run like a brace of scared antelopes in the Ganges jungles, but unless the boys have used their layoff time to dope out some defence against that Plaxton-to-Capirchio passing game they’re as doomed as a sitting duck.
2-: From Wise Guys Name Wallace to Butcher Socking Sam, by Gordon Sinclair, published in The Toronto Daily Star (Toronto, Ontario, Canada) of Tuesday 16th March 1937 [page 13, column 1]:
The foxy eyed gentlemen […] had been watching Gordon Wallace, Canada’s hard hitting welterweight champ, thud punches at the bag, and they were feeling sorry for Sammy Luftspring who is to fight him for keeps in two weeks.
[…]
In the whole group there was nobody to defend Luftspring. He was doomed like a sitting duck.
3-: From Britains’ Biggest War Since 1918!, by Gordon Sinclair, published in the Toronto Star Weekly (Toronto, Ontario, Canada) of Saturday 29th May 1937 [General Section, Number Two, page 3, column 2]:
Suppose you are too weak, too poor or just too lazy to build such an armed home for yourself. To live without one means death because any rival tribesman who comes upon you reaches for his gun or his great knife and you are doomed like a sitting duck.
4-: From a review of Star-Begotten : A Biological Fantasia (New York: The Viking Press, 1937), by the British novelist Herbert George Wells (1866-1946)—review by J. G. E. Hopkins, published in the Catholic Advocate (Brisbane, Queensland, Australia) of Thursday 19th August 1937 [page 5, column 1]:
From page 12 to page 30 of “Star-Begotten,” Mr. Wells shoots a few sitting ducks, that is, he destroys the Fundamentalist Protestant position as exemplified in the beliefs of his hero. Then, having reduced the man, Joseph Davis, to a state of neurotic agnosticism, he plants an idea in his mind, permits the idea to run its devastating course, introduces some shrewd and witty satire on current affairs and concludes with a happy ending, full of sunny optimism for the future of the race.
5-: From On The Spoor Of The Great, by ‘The Dustman’, published in the Daily Telegraph (Sydney, New South Wales, Australia) of Saturday 11th September 1937 [page 6, column 1]:
Mr. Richard Curle is a cosmic autograph-collector, an inspired lion-hunter, for whom life is one long safari of celebrities, to be tracked down, touched if possible, and tagged.
[…]
He bearded and bored Meredith in the old gentleman’s eightieth year, he ran Watts-Dunton to earth in the hope of having a squint at Swinburne, then measuring out his life at Putney Hill with brandy and bad verse.
[…]
Another sitting duck added to the Curle bag was poor old William Rossetti. “He was 72 when I first met him,” confesses Mr. Curle. “It was through my brazen cheek in writing to ask him for some autographs of his family and circle.”
6-: From The Springfield Union (Springfield, Massachusetts, USA) of Monday 13th June 1938 [page 9, column 7]:
New York, June 12 (AP)—In the important matter of picking the winner of the 15-round heavyweight championship fight between Joe Louis, the Negro, and Max Schmeling, the German, June 22, something to keep in mind is the fact that Max is making the supreme gamble in the history of pugilism.
[…]
Schmeling would not be going into the ring unless he was absolutely convinced of the outcome. He thinks it will be like shooting a sitting duck.
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