This is the 16281st most frequent Polish word.
usteckiej
"Usteckiej" is the adjectival form meaning "of or pertaining to Ustka," a town in Poland.
Here, 'Usteckiej' is a genitive case form of 'Ustecka', an adjective derived from 'Ustka', and it agrees with 'Biblioteki', a feminine noun in Polish, to indicate the library located in or associated with the place Ustka.
Idziemy na wycieczkę do Usteckiej Biblioteki.
We are going on a trip to the Usteckiej Library.
In this sentence, 'usteckiej' describes the term 'okolicy', which is feminine and in the genitive case, referring to an area related to Ustka.
Mamy mapę usteckiej okolicy.
We have a map of the Usteckiej area.
Here, 'usteckiej' modifies 'ryby', which is in the genitive singular form, showing possession or origin, referencing the fish characteristic or originating from Ustka.