This is the 16550th most frequent Polish word.
wiêzieniu
"In prison" or "imprisonment."
The word 'więzieniu' is used in the locative case, indicating the location of the subject (uncle's years spent).
Mój wujek spędził kilka lat w więzieniu.
My uncle spent several years in prison.
Here, 'więzieniu' is used in the locative case, referring to the abstract concept of prison as a place.
Tomek zawsze bał się myśli o więzieniu.
Tomek always feared the thought of prison.
In this sentence, 'więzieniu' is again in the locative case, describing the location where the programs take place.