This is the 14455th most frequent Turkish word.
oturmadığı
"That he/she/it does not sit" or "that it does not fit."
Here, 'oturmadığı' describes the chair as one not used for sitting by her, being part of a relative clause modifying 'sandalye'.
Bu sandalye onun oturmadığı bir sandalye.
This chair is one that she does not sit on.
In this example, 'oturmadığı' refers to a location that the subject does not sit in, used adverbially to describe the context of speaking.
Oturmadığı yerden konuşmak alışkanlık haline geldi.
Speaking from a place he/she does not sit in has become a habit.
Here, 'oturmadığı' signifies a place where she does not reside, providing meaning within a relative clause describing 'bir yer'.