It is when you literally do not care, and cannot be bothered to to shout at the person who has annoyed you. You do not wish to personally offend them so using this subtle but effective acronym will definitely suffice.
Mainly used between female-male conversations, used by teenagers most of the time.
Person 1: Heyy! do you remember when James cheated on me?
Person 2: no....
Person 1: REALLY?! well right i wa-
Person 2: OMG STFU IDEC.