>To be an easy target
>To be in a vulnerable position to be attacked
This phrase originated from the early days of hunting ducks. When the birds were in flight, they were much harder to kill. But when they are still, sitting on a lake, then it was easy for the hunters to target and kill them. So the phrase sitting ducks came to be used to refer to the state of being an easy target to be attacked.
Example
I hate to feel like a sitting duck in any situation.
The fact that she had no backup plan in case she got caught made her a sitting duck