sitting duck
noun [ C ] uk/ˌsɪt.ɪŋ ˈdʌk/ us/ˌsɪt̬.ɪŋ ˈdʌk/
活靶子；易被击中的目标；易受攻击的对象
someone or something that is very easy for an enemy to shoot or attack
With their bullets all gone, the soldiers were sitting ducks for the enemy. 子弹全打光了，战士们成了敌人的活靶子。