This is the 880th most frequent Croatian word.
bivšeg
"Former" or "ex".
Here, 'bivšeg' is used as an adjective to describe a noun, 'kolegu', indicating a past or former relationship.
Vidio sam bivšeg kolegu na ulici.
I saw a former colleague on the street.
'Bivšeg' in this context refers to something previous or past, modifying 'posla' to describe a past employment position.
Razgovarao sam s bivšeg posla jučer.
I talked with someone from the former job yesterday.
In this sentence, 'bivšeg' is an adjective modifying 'zida', pointing to the wall's state as no longer standing or existing.