This is the 9710th most frequent Polish word.
zamościu
"Zamościu" is the locative case of "Zamość," referring to being in or about the town of Zamość in Poland.
The word 'Zamościu' is inflected into the locative case, which is used after prepositions such as 'w' (in) to indicate location in Polish.
Byłem kiedyś w Zamościu.
I was once in Zamość.
Here, 'Zamościu' is used again in the locative case because the preposition 'w' (in) governs locations and requires this grammatical construction.
Nie każdy mieszkaniec w Zamościu zna jego historię.
Not every resident in Zamość knows its history.
In this sentence, 'Zamościu' might be interpreted metaphorically or poetically, emphasizing the emotional closeness to the location, again using the locative case structurally.