This is the 1909th most frequent Polish word.
sienkiewicza
"Sienkiewicza" is a possessive form referring to "Sienkiewicz," a common Polish surname, often associated with the famous writer Henryk Sienkiewicz. It means "of Sienkiewicz."
Here, 'Sienkiewicza' is used in the genitive case to indicate possession, referring to something (a book) belonging to Sienkiewicz, a Polish author.
Czytałem książkę Sienkiewicza w zeszłym tygodniu.
I read a book by Sienkiewicz last week.
In this context, 'Sienkiewicza' is used as part of a street name, again in the genitive case, which is common for place names derived from personal names in Polish.
Spotkaliśmy się na ulicy Sienkiewicza.
We met on Sienkiewicz Street.
This usage places 'Sienkiewiczowi', a different inflected case (dative), to indicate to or for Sienkiewicz in a dedicatory sense.