The idiom sitting duck seems to have appeared in Britain and in North America at about the same time. We will examine the definition of the expression sitting duck, where it came from and some examples of its use in sentences.
A sitting duck is someone or something who is an easy target, someone or something who is defenseless or vulnerable. The idiom sitting duck was first used by the British and the Americans during World War II to refer to military targets that were hard to defend, such as foot soldiers without places to retreat to from enemy fire. The term sitting duck was borrowed from the sport of hunting, as a duck that is sitting in a marsh or on the water is easier to hit than a duck that is in flight. Today, the term sitting duck may refer to someone who is literally a target for a bullet, but is most often used in a figurative sense. World War II was the source of many English idioms, as an idiomatic expression is a quick way to communicate ideas in a few words. As these slang phrases rise in popularity, they make their way into the mainstream English vocabulary and into the dictionary.
The resulting haphazardness of its patterns of development, we may imagine, make the city a sitting duck for all sorts of disasters. (New York Magazine)
Andrew Lapin, 28, a freelance journalist on his way to Lakeview to meet friends for dinner, said he felt “trapped” on the train, not clear what happened a few train cars away and worried he was a sitting duck if the shooting resumed. (The Chicago Tribune)