This is the 8043rd most frequent Polish word.
niedzielskiego
"Niedzielskiego" is the genitive form of the Polish surname "Niedzielski," typically meaning "of Niedzielski" or "related to Niedzielski."
Here, "Niedzielski" functions as a surname in its nominative case to indicate a person's name.
Pan Niedzielski zaprosił mnie na spotkanie.
Mr. Niedzielski invited me to a meeting.
In this sentence, "Niedzielskiego" is used in the genitive case to indicate possession.
Czy widziałeś teczkę Niedzielskiego?
Have you seen Niedzielski's folder?
"Niedzielskim" is in the instrumental case after the preposition "z" (with), referring to the person one conversed with.