This is the 13706th most frequent German word.
einstigem
"Former" or "previous."
Here, 'einstige' refers to something that existed in the past, showing its transformation into something else.
Das einstige Schloss wurde zu einem Museum umgebaut.
The former castle was converted into a museum.
In this sentence, 'einstigem' is used to describe 'profession' as something pertaining to the past.
In seinem einstigen Beruf war er Ingenieur.
In his former profession, he was an engineer.
The term 'einstige' here pertains to 'school' as an entity belonging to a previous time in the past.