This is the 6674th most frequent Polish word.
duńczyk
"Dunńczyk" means "Dane" (a person from Denmark) in English.
Here, 'Duńczyk' is declined as 'Duńczykiem' indicating it is in the instrumental case to describe Jan's nationality.
Jan jest Duńczykiem.
Jan is a Dane.
In this sentence, 'Duńczycy' is in the nominative plural form and refers to the Danish people collectively.
Duńczycy słyną z wysokiej jakości życia.
The Danes are famous for their high quality of life.
Here, 'Duńczyk' is declined as 'Duńczyka' and in the accusative singular form, used to describe the author of the book.