This is the 15795th most frequent Polish word.
sąsiadka
"Neighbor" (female).
Here 'sąsiadka' means 'female neighbor', referring to the person living nearby. It is used in the nominative form.
Moja sąsiadka zawsze przynosi świeże ciasto.
My neighbor always brings fresh cake.
The word 'sąsiadką' is in the instrumental case as it denotes the person with whom the action ('rozmawiałem') is being performed.
Rozmawiałem z sąsiadką o planach na weekend.
I talked with my neighbor about plans for the weekend.
In this sentence, 'sąsiadkę' is the accusative form because it is the direct object of the verb 'spotkałem'.