From Longman Dictionary of Contemporary Englishsitting duckˌsitting ˈduck (also sitting target) noun [countable] ATTACKsomeone who is easy to attack or easy to cheat Out in the open, the soldiers were sitting ducks for enemy fire.
Examples from the Corpus
sitting duck• This makes the whole colony a sitting duck for a man with a gun.• Being left in front, Morley Street looked a sitting duck approaching the line but scrambled home.• Raskin was proceeding gingerly, aware that Macintosh was a sitting duck for cancellation.• She was a sitting duck out here all by herself!• We were like sitting ducks for pickpockets in the city. 