This is the 8993rd most frequent Polish word.
sosnowieckiej
"Sosnowieckiej" is the genitive or locative singular form of an adjective meaning "of/from Sosnowiec," referring to something related to the city of Sosnowiec in Poland.
This usage refers to a location or name (proper noun), specifying that 'Sosnowieckiej' describes the street as belonging or related to Sosnowiec, a city.
Mieszkam na ulicy Sosnowieckiej.
I live on Sosnowiecka Street.
Here, 'sosnowieckiej' is used as an adjective describing the steel as coming from or related to Sosnowiec, a city, indicating origin or associative quality.
Odwiedziłem fabrykę sosnowieckiej stali.
I visited the factory of Sosnowiec steel.
'Sosnowieckiej' is used as an adjective modifying 'okolicy' (area), specifying that the area is related or nearby the city of Sosnowiec.