Green’s Dictionary of Slang

1. (orig. milit.) an easy target, someone or something vulnerable and defenceless, both lit. and fig.

1943		Smith & Carnes American Guerrilla 47: ‘Tell bomber command several hundred Axis trucks are going to be ducks, heading south in the desert’ .
1948		Harper’s Mag. Apr. 290: Men of the Senator’s type have been sitting ducks for the opposition.
1953		J. Michener Bridges at Toko-Ri 75: They’re sitting ducks! [...] Clobber those guys.
1965		H. Williamson Hustler 194: It was considered by hustlers a duck [f.n.] ‘cause it was on a dark corner, there usually wasn’t no peoples in sight, and the traffic was slow [f.n. Duck—a place that’s extremely easy to rob].
1966		B. Reckord Skyvers Act II: I’d be a sittin’ duck for the cops wiv that bike and you know it.
1971		Harper’s Mag. Feb. 89: A gambling house is a sitting duck to every con man or outlaw who comes through: he is invariably convinced that he has a scam that you have never seen before.
1981		S. Berkoff Decadence in Decadence and Other Plays (1985) 37: Snap my fingers, I’ll find some more / they’re sitting ducks.
1990		(con. early 1950s) J. Ellroy L.A. Confidential 5: Sitting ducks: two men, three triple-aught rounds close in.
1999		Indep. on Sun. Rev. 20 June 30: Lonely woman, out in the sticks somewhere, no car, yearning for love, sitting duck.
2000		Indep. Rev. 22 Jan. 10: ‘Terry’ was a sitting duck.
2007		Post-Crescent (Appleton, WI) 16 Nov. 49/3: He’s [i.e. a football quarterback] a sitting duck in the pocket and like Bollinger, the arm strength is suspect.
2016		Dly Advertiser (Lafayette, LA) 20 July A3/1: Iberia sheriff says he is a ‘sitting duck’ without a gun.

2. (US) a parked stolen car.

1970

J. Wambaugh New Centurions 44: ‘How often you pick up a sitting duck?’ asked Serge [...] checking a license plate against the numbers on the hot sheet.