This is the 12646th most frequent Polish word.
niczyją
"Niczyją" means "nobody's" or "belonging to no one" in English.
In this sentence, 'niczyją' is used as an adjective to refer to something that does not belong to anyone specifically.
Nie można znaleźć niczyją rzecz na ulicy.
You cannot find anyone's belonging on the street.
Here, 'niczyją' modifies 'książkę', meaning it describes an object that is not claimed by anyone.
Widzę niczyją książkę na stole.
I see an unclaimed book on the table.
In this example, 'niczyją' describes 'rękawiczkę', indicating that the glove lying on the sidewalk does not belong to anyone.