This is the 12645th most frequent Polish word.
nich
"Them" (third-person plural pronoun in the dative or accusative case).
The word 'nich' is used here to refer to a group of people in the genitive case after the preposition 'o'.
Nie mogę przestać myśleć o nich.
I can't stop thinking about them.
In this sentence, the first instance of 'nich' is used in the genitive case after 'o', which explains possession by the antecedent.
Podczas rozmowy dziwiłem się ich wiedzą o nich.
During the conversation, I was amazed by their knowledge about them.
Here, 'nich' is used in the genitive case after the preposition 'od', to denote origin or source.