"whose ass is it gonna be" or WAITGB is used when trying to figure out who is to blame in a situation, usually to punish them.
Person 1: Someone robbed a helpless elderly man.
Person 2: Alright, whose ass is it gonna be?
or it can be used jokingly
Person 1: Someone leaked a major spoiler for a highly anticipated movie.
Person2: Whose ass is it gonna be?