This is the 17315th most frequent Polish word.
zamojskiego
"Zamojskiego" means "of Zamojski" or "related to Zamojski" (a surname or something associated with it).
Here, 'Zamojskiego' refers to something belonging to or associated with Zamoyski, indicating possessive form in Polish.
Byłem w muzeum Zamojskiego.
I was at Zamoyski's museum.
In this usage, 'Zamojskiego' signifies the genitive case to indicate 'of Jan Zamoyski'.
Czytaliśmy wiersze Jana Zamojskiego.
We read the poems of Jan Zamoyski.
The word 'Zamojskiego' here also represents the genitive case, used to denote association or relationship.