This is the 5804th most frequent Polish word.
zakonu
"Zakonu" is the genitive or locative form of "zakon," meaning "of the religious order" or "in the religious order" in English.
The word 'zakonu' is used in the genitive case here to indicate possession, describing which order the monastery belonged to.
Klasztor należał do zakonu Dominikanów.
The monastery belonged to the Dominican order.
Here 'zakonu' is also in the genitive case and is used to specify the order whose history is being referred to.
Historia tego zakonu jest niezwykle inspirująca.
The history of this order is extremely inspiring.
In this sentence, 'zakonu' is again in the genitive case because it specifies which kind of member was encountered—one from a medieval order.