This is the 9501st most frequent Polish word.
wieluniu
"Wieluniu" is the locative case of "Wieluń," referring to the name of a town in Poland used to indicate "in Wieluń" or "about Wieluń."
Here, 'Wieluniu' is in the locative case, indicating the specific location 'in Wieluń'.
Byłem w Wieluniu na wakacjach w zeszłym roku.
I was in Wieluń on vacation last year.
Again, 'Wieluniu' is used in the locative case to denote location.
Podczas pobytu w Wieluniu odwiedziłem muzeum.
During my stay in Wieluń, I visited the museum.
'Wielunia' here is in the genitive case, used to indicate direction or origin.