This is the 16554th most frequent Polish word.
więzieniu
"In prison."
The word "więzieniu" is used in the locative case, indicating the location where the action is taking place.
On przebywa w więzieniu za swoje czyny.
He is in prison for his actions.
The word "więzieniu" is again in the locative case, used to specify the place that cannot be visited.
Nie możesz odwiedzić go w więzieniu.
You can't visit him in prison.
Here, "więzieniu" is used to denote the context of the discussion, indicating where these conditions exist.