Example: Salesmen learn to size up a prospective customer. When they see a sitting duck, they know it instinctively.
Origin: A duck on the ground, a "sitting duck", is far easier to shoot than a flying duck. It wasn't until this century and in some states well into this century that it became illegal to shoot ducks on the ground.
Reader's Digest had the phrase as part of a title in 1944: "Why Tankers Are No Longer Sitting Ducks." In 1949, the expression was explained as being derived from the fact that marksmanship in duck hunting is determined by the ability to hit ducks in flight and not while they are sitting.