An idiom, meaning "vulnerable", "an easy target", "a person who is easy to deceive".

Origin: A duck on the ground, a "sitting duck", is far easier to shoot than a flying duck. It wasn't until the 19th century and in some states well into the 20th century that it became illegal to shoot ducks on the ground.

When hunting, it's quite normal to use "replicas" of the bird you hunt on the ground to attract other birds of the same kind. They seem to think something in the line of "Heeey, there's a bunch of ducks down there! Let's camp there tonight!" The birds will come in to land, and the hunter will unleash a hailstorm of lead upon them.