This is the 866th most frequent Turkish word.
birini
"Birini" means "someone" or "somebody" in English, often used to refer to a specific person in an object position.
The word 'birini' is the accusative form of 'biri', meaning 'someone' with a direct object case.
Birini arıyorum.
I am looking for someone.
Here, 'birini' indicates the direct object of the verb 'to upset'.
Birini üzmek istemem.
I don't want to upset someone.
In this case, 'birini' highlights the individual being known as the direct object.