This is the 6309th most frequent Polish word.
borussii
"Borussii" is not a standalone Polish word but resembles a Latin genitive form meaning "of Prussia." In Polish, it could relate to historical or geographical contexts involving Prussia.
Here, 'Borussii' is used as the genitive case form to imply possession, showing the relation of the fans to Borussia.
Borussii kibice śpiewali na stadionie.
Borussia's fans were singing at the stadium.
In this sentence, 'Borussii' is the genitive case, referring to the club Borussia as the entity whose players are being discussed.
Piłkarze Borussii zdobyli kolejne zwycięstwo.
The players of Borussia achieved another victory.
Here, 'Borussii' is again the genitive case, denoting association or belonging of the player to Borussia.