This is the 9821st most frequent Polish word.
zygmunt
"Zygmunt" is a Polish given name equivalent to "Sigmund" or "Sigismund" in English.
In this sentence, 'Zygmunt' is used as a proper noun, referring to a person's name, the king of Poland.
Zygmunt był królem Polski.
Zygmunt was a king of Poland.
Here, 'Zygmuncie' in the Polish locative case refers to Zygmunt in the context of discussing or reading about him.
Muszę przeczytać książkę o Zygmuncie.
I have to read a book about Zygmunt.
In this structure, 'Zygmunta' is in the genitive case, indicating possession (the painting belonging to Zygmunt).